Maud Spellbody, a young witch, is flying on her broomstick to Cackle's Academy for Selection Day. She crashes into Mildred Hubble's balcony. Mildred helps Maud, whose glasses were broken in the crash, get to the school. A new student there, Ethel Hallow makes a good first impression on the teachers. The girls tell Miss Hardbroom that Mildred was supposed to attend Pentangle's, but decided Cackle is better. Esmerelda Hallow, Ethel's older sister, gives the students a tour and Miss Cackle helps Mildred cope with the effects of a transfer spell. When Mildred and Maud sneak away, Ethel eavesdrops and learns Mildred is not from a magical family. Later, Mildred struggles with the written exam. Miss Cackle encourages her to do what she is good at. Being good at art, she draws pictures instead of writing answers, and only receives a 1% grade. Miss Cackle gives her twin sister, Agatha, permission to speak to Miss Hardbroom for an internet article. The students make potions and Mildred successfully levitates, after taking some of Ethel's pondweed from her dragon potion (due to Ethel having taken all of it), wrongly turning Ethel into a worm.

First appearances: Bella Ramsey as Mildred Hubble, Meibh Campbell as Maud Spellbody, Jenny Richardson as Ethel Hallow, Miriam Petche as Esmerelda Hallow, Tallulah Milligan as Drusilla Paddock, Dagny Rollins as Felicity Foxglove, Clare Higgins as Miss Ada Cackle/Agatha Cackle, Raquel Cassidy as Miss Hecate Hardbroom, Wendy Craig as Miss Bat, Shauna Shim as Miss Drill, Kacey Ainsworth as Miss Gullet, Zita Sattar as Miss Tapioca and Nicola Stephenson as Julie Hubble

The first years try out their magic tablets "maglets" and Enid arrives at school with her famous parents. Maud is sent to Miss Cackle for drawing a funny picture of Miss Hardbroom and Ethel. Miss Cackle allows Maud to convert a space under the stairs for her class. Mildred is amused by Enid singing out of tune and Enid picks Mildred to look after her whilst Ethel swaps her maglet with Maud's. Mildred ignores messages she believes are from Ethel. Enid shows Mildred her pet monkey, Muddles. Maud is hurt that Mildred is spending time with Enid and Muddles escapes, but who ends up in Miss Cackle's study. In potions, Mildred and Enid are cauldron partners and they make an invisibility potion instead of a laughter potion. Mildred attempts to make friends with Maud by, but she unknowingly reveals hers and Enid's plan to get Muddles out of Miss Cackle's study over their maglets. Mildred and Enid get to the study and Enid's monkey is actually a cat and Maud and Drusilla help Mildred and Enid escape. Miss Hardbroom is annoyed with Ethel when the room is empty. Maud confiscates Ethel's maglet and Enid makes friends with Maud.

First appearance: Tamara Smart as Enid Nightshade

Mildred and Enid jump on the fields, which they have made bouncy, not caring about studying for spell science with Miss Gullet. Mildred receives her first ever toy, a teddy named Puss, from her mum in the post. In spell science, Mildred is asked to turn a rat into a frog and Maud refuses to help. Mildred turns Ethel into a pig and when Miss Hardbroom finds out, she gives Mildred until later that day to perform a simple transformation spell or she will leave the school. Mildred decides to leave of her own accord, but is tricked into meeting Ethel by Drusilla and Ethel turns her into a frog. Mildred ends up by the pond and meets another frog, Algernon Rowan-Webb. Maud refuses to let Enid help her find Mildred, despite Enid reassuring her that she is Mildred's best friend. Mildred and Algernon get into the school and end up in jars in the potions lab. Maud reveals Ethel has done something to Mildred to the teachers and Enid finds out she is one of the frogs. Ethel is forced to turn her back to herself and Mildred reveals the identity of the other frog and it turns out Miss Gullet was responsible in order to get her job. Miss Gullet is forced to turn Algernon back to his human form and once she has done so, she is dismissed. Algernon persuades Mildred to take the test and she performs a multi-transformation spell, impressing Miss Cackle and Miss Bat. Ethel and Miss Gullet are turned into frogs and Mr Rowan-Webb becomes the new spell science teacher.

The Great Wizard visits Cackle's Academy and he meets the pupils. The Great Wizard talks through recent incidents caused by Mildred and when The Great Wizard finds out Mildred is not from a witch family and Miss Cackle defends Mildred against the Great Wizard. Mildred has to do a magic display for The Great Wizard. Ethel shows The Great Wizard the school trophies and Ethel tells The Great Wizard about Mildred. Enid tells Mildred she suspects Ethel wrote the letter to The Great Wizard, after overhearing her talk to him. It turns out Agatha is pretending to be Miss Cackle, as Miss Cackle is in a deep sleep. Mildred performs her magic display, a beaker and bottle band, but she messes up when the potions explode over the staff and The Great Wizard insists Miss Cackle leaves. He allows Miss Cackle to stay after the staff persuade him and Mildred is chosen to be the lead in a broomstick display for The Great Wizard and Ethel has to lend her broom to Mildred. Mildred reveals to Maud that Miss Cackle is Agatha when she tries to perform a spell on Mildred and Maud wakes Miss Cackle up. Miss Cackle proves she is the real Miss Cackle by saving Mildred and Enid from crashing.

The pupils call home and Ethel is hurt when her mum can't spare the time to speak to her, after spending ages talking to Esmerelda. Ethel is horrible to Mildred, making remarks on how she is always bottom and Mildred decides she won't be any longer. Enid suggests Mildred does a wisdom spell and during the night, Mildred takes Miss Bat's socks for the spell and after thinking it was goblins, Miss Cackle and Miss Hardbroom decide to give her some time off. Mildred makes the spell, but thinks it hasn't worked, however, the following morning, Maud finds Mildred looking old. Maud tries to look for a reversal spell, but Esmerelda informs her that only time can undo a wisdom spell. Maud encounters Miss Darkside, the supply teacher who is not nice, and she takes her to Mildred, pretending she is Miss Cackle and to tell Miss Darkside she is no longer required. Miss Cackle mistakes Mildred for Miss Darkside and Mildred has to teach her class, pretending that the absence of her younger self is because of her mum's toenails and she sends Ethel and Drusilla out of class, impressing Miss Hardbroom. Mildred gets the class to act out a story. Ethel and Drusilla phone Miss Hubble and they find out nothing is wrong with her and Ethel takes the information to Mildred. After Miss Cackle decides to phone Miss Hubble, Mildred decides to admit what's happened, but the spell wears off as she enters Miss Cackle's office. Miss Cackle magics Miss Hubble to her office and Maud makes an ingrown toenail spell. Mildred gets the result from a test, where she placed second bottom. Ethel tells Mildred she impersonated a teacher so she tells Ethel she phoned her mum, and Ethel walks off, and Mildred and Maud think Mildred's mum is a witch as the spell has worked on her.

The parents visit the school to have a talk with the teachers on how their children are doing. Maud wants her parents to be proud of her, so she makes everyone forget about her mistakes. The problem: Mildred forgets her as well, since all of her mistakes where only due to Mildred. So she decides to not be a flawless pupil but to get her best friend back. His father is erasing the memories of the teachers as well, and everything turns out good. Her parents are still proud of her, even though she is not perfect.

Mildred and Ethel are the chosen ones for a competition against another school in potion making. Mildred is told to do nothing at all under the competition, which makes her a little sad. The headmistress of the other school, Miss Pentangle, knows how it is to be overlooked, and she offers Mildred a place at her school—she sees her potential. At the end of the competition, she finds a rule that activates Mildred in the potion making. Mildred does great, as Miss Pentangle had expected. Mildred chooses not to leave Cackle's Academy, but is grateful for the offer. Miss Pentangle and Miss Hardbroom were friends once, and Mildred helps both of them to become friends again.

Mrs Cackle is barricading the school after it has been surrounded by mist. It is the mist of time, created by a mighty sorcerer who afterwards got lost in "the mists of time". Ethel's cat is gone missing outside, so Mildred helps her getting outside the castle, searching for her cat. They do not find the cat, and the re-enter the castle a different way than they left it. If they would have re-entered the same way, they would have been back in their own time. But now they have traveled back in time. They meet Mona (Maud's grandma) and the Cackle twin sisters, Agatha and Ada. Mildred finds out that Ada (the regular Miss Cackle) is not the oldest one and therefore should not have been the head of the school in the future. Furthermore, she figures out that Ada is not a good witch and tries to steal her twin sister's power. Suddenly, the old Mrs Cackle enters the scene and helps Mildred to make herself a non-naughty witch and fixing the past by not expelling Mona. Both Mildred, Ethel, and Ada find their way back to their own time. Mildred concludes that she might become the best witch, since even Ada, the naughtiest witch, has become the best witch.

Absent: Zita Sattar as Mrs Tapioca

Agatha takes over the school as headmistress. Miss Gullet installs a detection spell on the entire school to make an alarm when unauthorized magic is used. Agatha claims that Ada Cackle has run away, but actually, she has been trapped in a living painting of herself by Agatha. Ethel and her sister Esmeralda discuss what to do about The Great Wizard being called to visit the castle as he will freak out. Ethel feels guilty and summons Miss Gullet, who transforms Esmeralda into a trophy. Ethel sends Mildred's cat Tabby to Mildred's home, with an invisibility spell and tells her she needs to come and save the castle – Mildred uses a kitchen sweep instead of a broomstick and decides to go. Mildred breaks into Cackle's office where the painting of the Cackle sisters is, finding Maud, Enid and Ethel trying to help Ada, but Miss Gullet enters and transforms the visible girls into cakes for a great feast in the main hall, while Mildred's invisibility wears off. The Great Wizard arrives, and hears of Agatha's evil doings via Mrs Bat and Mr Rowan-Webb. Miss Gullet sees them grassing up and transforms them into frogs. Agatha, Miss Hardbroom and Miss Gullet transform the Great Wizard into a balloon. Miss Hardbroom tries to get Ada out of the painting, but Miss Gullet and Agatha hear the unauthorised magic alarm and send Miss Hardbroom into the painting as well. Meanwhile, Mildred stops the witches transformed into cakes from being eaten by the other girls. Mildred rises up against Ada and asks all the girls to vote, agreeing that none of the girls will stay at the school if Agatha is headmistress, so Agatha casts an Annihilation spell to destroy the castle and everyone in it. Miss Drill attempts to hold it back while the girls escape. With the castle crumbling down, Mildred starts chanting and brings all the girls into a circle and casts an incredibly powerful spell that reverses all of Agatha's evil magic and turns everyone back to themselves again. The Great Wizard traps Agatha and Miss Gullet into a photograph of them both, which Ada hangs up in her office for the rest of time. Mildred just scrapes by on her exams with 51%, ending the school year.

Mildred makes her way to school for her second year feeling positive and impresses Maud and Enid with her flying. On her way, her school project is blown away and Ethel accompanies her younger sister Sybil for her first day. Mildred's project lands on Ethel and new first years Clarice Twigg and Beatrice Bunch meet each other. Mildred finds Ethel with her project on making animals talk on the roof and Mildred slips off the roof. Hanging on by a flag pole, Sybil summons Mildred's broomstick to her, but part of the school falls down when Mildred casts a spell to help get her down and the staff find out some of Agatha's bad magic was left behind. Clarice is made head of the first year and she and Beatrice make friends with Sybil when they have to share a room. Mildred hides her talking tortoise and project whilst Ethel has lost hers. Maud tells Mildred and Enid that her physical appearance changed during a game of magic hide and seek. Beatrice gives Sybil a confidence potion and when Ethel has a go at Sybil for not packing her project, Sybil stands up to Ethel. The second years present their projects to Miss Hardbroom and Mildred is frustrated when Ethel does a spell on making animals talk, which Ethel stole from Mildred and when Mildred replicates the spell to prove Ethel stole it, Miss Hardbroom sends Mildred to Miss Cackle. Mildred ends up on the roof with the staff, who find the founding stone and Sybil rescues Mildred's tortoise from a high tree, who hand it over to Maud and Enid to prove Mildred did her project on talking animals. Beatrice reveals to Sybil that the potion was just water and Miss Cackle shows off the founding stone to the school. The tortoise tells Miss Cackle and Miss Hardbroom what Ethel did and they find Mildred's project in the bin, like the tortoise said and Mildred wins a prize for her project.

First appearances: Trixie Hyde as Sybil Hallow, Kitty Slack as Clarice Twigg and Ynez Williams as Beatrice Bunch

Everyone is woken by a fire drill and Mildred sees Miss Hardbroom on fire, so she throws a bucket of water over her. When Mildred and Ethel insult each other, Miss Cackle leaves an orange handprint on their backs. Whilst the students observe the founding stone, Mildred and Ethel find out about the handprint when Ethel temporarily turns Mildred into stone. After surveying a package outside, Miss Bat opens it, which turns out to be stone, but later, eyes appear. Mildred and Ethel become joined together by the arm, which Miss Cackle reveals is a friendship trap that will only undo when they make friends and Sybil refuses to leave her room after recent events. In Art, the stone turns into the new teacher, Miss Mould and when Mildred is complimented on her clay work, Ethel turns it into snakes and when Miss Mould changes the spell, they class are covered in clay and erupt into laughter, which Miss Hardbroom is unhappy with. Beatrice's asthma transports her and Sybil to different places around the school whilst the friendship trap begins to wear off between Mildred and Ethel when they talk about Esmerelda having no magical powers. The founding stone ends up in Sybil's possession and to return the stone, Clarice suggests they start a fire to set off the alarm to enable them to return the stone. When the staff find out the stone is missing, they organise a search. When the fire alarm goes off, Sybil is scared and Mildred helps Ethel in reassuring her and the friendship trap breaks. Miss Hardbroom confiscates Sybil, Clarice and Beatrice's cats for the fire. Ethel refuses to help Sybil return the founding stone, but Mildred agrees to.

Miss Pentangle arrives at Cackles Academy to run some modern magic workshops, with students speculating she is the new headmistress. Mildred, Maud and Enid are upset about the prospect of all going to different schools as Maud's parents are visiting Miss Amulet's Academy and Miss Cackle continues to look at the petition. Miss Pentangle is summoned to Miss Cackle's office and admits she has been asked to be headmistress at Cackles as well as Pentangles. Miss Pentangle impresses the first years with her modern chanting and gives Mildred a power boost in order to perform an invisibility spell in potions. Sybil faints in chanting after performing a chant with confidence and Mildred confides in Miss Pentangle over wanting to get Miss Cackle reinstated. Mildred sneaks off to Miss Cackle's office during chanting to see the petition when her classmates say their parents never signed and Miss Pentangle watches Miss Cackle help Sybil with her spells and potions when she becomes emotional after struggling. With Maud and Enid's help, Mildred makes a true author potion when Maud and Enid's signatures are on the petition and when they attempt to show Miss Hardbroom, Mildred burns the scroll as her powers are strong. The Great Wizard arrives at Cackles and Mildred tells him she believes the petition signatures are forged, so using the mists of time, they find out Mrs Hallow forged the signatures. Mrs Hallow is magicked to the school and Ethel is hurt that Mrs Hallow did it out of revenge for Esmerelda and she is sacked from the council while Miss Cackle is reinstated.

(All Hallow's Eve) Esmerelda attempts to give the powers back to the stone, but fails. Miss Cackle allows the pupils to play magical tricks on the staff without fear on Halloween. Esmerelda and Sybil recall a bedtime story from when they were younger about one of their ancestors reigniting her coven's stone, so using the Hallow family tree, Mildred asks for the Hallow responsible for that to come forward and tell them what to do, but they fly away. Miss Hardbroom struggles to get Miss Mould's references from the school she used to teach at while Mildred, Maud, Enid and Ethel try to get the answers out the Hallow ancestors. When they find the Hallow who reignited the stone, the Hallow tells them she just took credit for it. Clarice and Beatrice are convinced the magic in the school has gone haywire and eventually Esmerelda and Sybil are forced to admit that Esmerelda has got her magic back by taking it from the Stone. When Miss Drill notices the school is freezing, Miss Cackle evacuates the school, but she, Miss Hardbroom, Maud, Enid and Felicity are trapped in the potions lab. Mildred returns to the family trees to get answers from the Hallows and the witch who actually reignited the Stone long ago emerges, who turns out to be Mildred's ancestor, Mirabelle Hubble.

(The Big Freeze) - In the season finale, Mirabelle explains that she had to sacrifice her magic as well as 12 descendants' magic to save the stone, and Mildred is her 13th descendant, the first magical Hubble for centuries. Mildred is delighted to find out that she is from a witching family after all.

Whilst Mildred's mum Julie is in the park, she is told by her miniature self to go to Cackle's. In the potions lab, Felicity is suspended in a block of ice, so Miss Cackle, Maud and Enid give the rest of their magic to Miss Hardbroom to help Mildred, putting them into ice blocks.

Mildred is unhappy about leaving the school, but Miss Mould tells her about a coven Mildred could go to, where she could be individual and creative. When Miss Hardbroom gets outside, it turns out that the founding stone is still in the school and the coven Miss Mould knows off is run by Agatha.

Miss Mould's plan was to allow Agatha to escape the photograph once the Stone's magic failed. Mildred praises Miss Mould as a teacher before she is frozen herself. Ethel decides to sacrifice her magic to the stone, but she is too late as when she makes a speech, Miss Hardbroom, Esmerelda and Sybil are put into ice blocks, and then she is.

Julie Hubble arrives at the school and gets Mildred out of her block; Mildred explains to her mum about their ancestor. Mildred decides to sacrifice her powers to the stone, but although she starts the process, Miss Mould, moved by what Mildred said to her, completes the ritual in her place and reignites the stone, losing her powers. The school is saved and later celebrates Halloween.

As the festivities unfold, Miss Cackle gives a brief, but emotional, speech to the rest of the school that while there's stuff needed to be cleaned up, they will celebrate that they are not just students and staff anymore, that they're a family.

The season ends with the school applauding Miss Cackle and Mildred, for their heroic deeds.

After Mildred uses the stolen Wishing Star to give her mother magic, Julie has no idea how to control her amazing new powers. Meanwhile, Beatrice's attempts to learn about cooking prompt her to discover that items are being stolen from the dining hall. It also emerges that Beatrice is hiding a secret about her mum, and ends up having a heart-to-heart with Miss Tapioca. Finally, Miss Tapioca's daughter, Mabel, joins Cackles.

With half the school including Ethel, Miss Cackle, Maud and Enid gone, Julie's magic spins out of control and Mildred learns from Miss Hardbroom that she wished her non-magical friend (Indigo Moon) magical too and she got addicted and it and it ended with her being turned by stone into a statue so its all up to Mildred to make sure the same won't occur to her Mum. Mildred and Miss Hardbroom visit Mildred's family tree where they find out how to reverse her Mum's gradual transformation into stone. Mildred brews the potion just in time and not just her Mum, but all the missing people are saved. Miss Cackle and Miss Hardbroom make her Mum realise that she has to give up her magic, which she does, and her Mum has to leave Cackles. Miss Hardbroom refuses to use what's left of the potion to save Indigo, not wanting to break the Witches Code like she did as a child. Elsewhere, Beatrice, Sybil and Clarice wake up on the morning after Sybil's party and accidentally turn Miss Drill's cat into a cake. This starts off a chain of lies that results in Miss Drill thinking she's been fired. Eventually, Mildred's potion restores the cat and Miss Drill agrees not to punish the girls for their mistake, but she will for throwing a secret party!

When Indigo disappears after being humiliated by Miss Hardbroom again on her first day at Cackles, Mildred and Maud track her to a TV talent show in a nearby city and wangle their way into the studio. Meanwhile, Enid falls out with Maud and Mildred saying Indigo is wasting her time.

With Miss Bat and Mr Rowan-Webb's Wedding approaching there is still no sign of Indigo Moon. When a wedding gift of a Wishing Star arrives Mildred and Ethel hatch a plan to steal the wish to help find Indigo. However, when Ethel is generously granted the making of the wish by Mildred, she enacts her real plan which is to summon a false Indigo with the intention of destroying the Academy - which Ethel can then thwart, thus restoring her family name... Until the false Indigo appears again and Ethel loses control of her. Only when the real Indigo turns up, will they be able to save the day. But with the real Indigo still missing, the school only have 30 minutes - hand over Miss Hardbroom who will be turned to stone as revenge for what happen to Indigo, or the false Indigo will destroy Cackles!

Final appearances: Philip Martin Brown as Mr. Rowan-Webb and Wendy Craig as Miss Bat

When an apparently botched potions experiment leaves Mildred with a whole new look, can she complete three impossible challenges and reverse the spell before the sun goes down? It is revealed this was sabotage by Ethel Hallow, who had substituted an appearance-spell to prevent Mildred from applying for head girl. Meanwhile, Beatrice, Sybil and Clarice have become part of the junior coven and are tasked by leader Fenella to prove their loyalty by teaching first year Izzy Jones (Peppercorn in the end credits) how to ride a broom. Izzy is from a non-magical background and it's clear very quickly that she has a lot to learn and that she's afraid of heights. Not knowing this, Beatrice attempts to help Izzy by giving her a levitation spell. Izzy lifts up in the air just before a shocked Clarice realises that nobody has taught her how to control the broom! Mildred becomes stuck in her new look when she fails to complete the last impossible challenge. Instead she rescues Izzy from her broomstick after falling off it. Fenella informs Clarice, Sybil and Beatrice that they're out of the coven after their failed attempt to help Izzy.

First appearances: Saoirse Addison as Izzy Jones and Billie Boullet as Fenella Feverfew

It is Indigo's birthday celebrations, and she suddenly starts ageing 30 years for a few moments at time. Maud thinks Indigo's lost time as a statue is reasserting her real age. Miss Hardbroom, who was trying to adopt Indigo, discovers Indigo has a real mother and returns Indigo back in time to be with her. She wipes Indigo's memory so she has no memory of Cackles or anybody there. Later, the adult Indigo arrives with her daughter, Azura, who has come to enrol at Cackles. Miss Cackle, with help from Felicity, makes Miss Hardbroom speak to Indigo and enrol Azura. This allows Miss Hardbroom to finally let go of her past with Indigo, relieved that it has all finally worked out. In one sub-plot, Miss Hempnettle, the new games mistress is introduced and it is emerged that she has a bad history with Miss Drill, who has picked up a sporting injury. Meanwhile, first year Izzy is struggling to settle in following her broomstick experience. In an attempt to help, Clarice, Beatrice and Sybil make a potion for Izzy to make Cackles seem more like home. But it only succeeds in making Izzy feel that she is surrounded by her family members and is really at home! She ends up in trouble after winding up Miss Drill and Miss Tapioca whilst under the influence of the potion. But she is persuaded to stay after Mildred and Sybil ask her to look after Azura.

First appearance: Luciana Akpobaro as Azura Moon

Clumsy Mr. Daisy, the new Spell Science teacher arrives at Cackles upsetting Mabel's sweet stall. Mildred is conned into visiting the "fortune telling" Crystal Lake by an evil old crone, Jessica Juniper, masquerading as a girl from Pentangles in the magic mirror. In the forest Mildred meets Clarice, Sybil and Beatrice, who were conned too. At the lake Mildred is attacked by the crone, and finally performs protection spell and is rescued by Maud and Miss Hardbroom. Feeling irresponsible she resigns as head girl contender, but, on seeing Ethel's future in Mr. Daisy's flask, she asks to rejoin, realising that if Ethel wins the head girl contest, there will be disaster for Cackles! Meanwhile, Enid is tricked by Miss Hempnettle into acquiring Miss Drill's special speed potion.

Enid leaves Cackles Academy after being tricked into stealing Miss Drill's speed potion formula. She rehearses with her eager mother for "The Deadly Nightshades and family" show. Miss Drill does not blame Enid and (along with Mildred) persuades her to take part in the Witch World Games qualifiers. Meanwhile, Miss Tapioca, Mr Daisy, and students rustle up ingredients for Miss Drill's potion which helps Enid to win, despite sneaky tricks by Miss Hempnettle. Enid accepts a place at the prestigious Mount Broom sporting academy now with the encouragement of her mother. Miss Drill is able to finally expose Miss Hempnettle as the cheat she is and has her summoned before the Magic Council. Enid says an emotional goodbye to Mildred, Maud and the rest of Cackles as she leaves for the sporting academy. In a sub-plot Ethel undermines Azura as a school ambassador to the Magic Council.

The arrival of a magical comet is announced. With Mildred and Ethel getting ready for the final head girl challenge (to create a new spell) Mildred starts to suffer from magical burnout. Knowing this Maud, Izzy and Mrs Cackle decide to send her to rest at Izzy's farm to watch the comet. Upon arriving Mildred's attempts at magic go wrong to the point of her magic completely failing. After a bit of digging it is found that Mildred needs her family's grounding stone to restore her magic, and since she has no idea where it is, Izzy's dad hatch's a plan to use the comet to create a new one.

Back at the school Ethel tries to disqualify Mildred from the head girl competition by snitching on a half-heard conversation between Mildred and Mr. Daisy when returning his sandtimer. Maud and Azura try to delay Ethel, this only works for a short while before Ethel brings Miss Hardbroom into it. This leads to a confrontation with Mr Daisy who ultimately protects Mildred by claiming he was the one to loop time. Miss Hardbroom announces that as a result, he will be fired at the end of term, leaving the girls devastated.

Ethel and Mildred perform their newly designed spells in the final round of the Head Girl competition. Mildred's spell apparently goes wrong and turns Headmistress Miss Cackle into shattered glass. Mildred is sent to The Correctional School for Undesirable Witches, Wormwood, where she is befriended by Camilla Clove (played by Madeleine Edmondson) and menaced by the cruel jailer Miss Splinter (played by Sue Vincent). Meanwhile, Maud analyses playback and realises someone else caused Miss Cackle's demise and deliberately gets herself sent to Wormwood too with a plan to free Mildred. This leaves Izzy and Azura to find out who framed Mildred. It turns out that Ethel was doing Agatha (Ada's evil twin) Cackle's bidding and tricked Felicity into sabotaging Mildred's spell. Izzy and Azura discover Felicity's role in the plan, who eventually decides to confess to Miss Hardbroom but Ethel stops her by lying that she will do it. Mildred and Maud make their escape from Wormwood and realize that they have to stop Ethel from becoming Head Girl.

In a sub-plot, an attempt to comfort (the soon to be dismissed) Mr Daisy with a Happiness Potion goes wrong, when he dances in an overly familiar way with an unimpressed Miss Hardbroom!

In the series finale, Agatha Cackle, impersonating headmistress Ada, promotes Ethel to head girl, and at the school assembly tricks everyone else into the Vanishment plane of existence, including Julie (who had been messaged by Maud). Agatha locks Mr. Daisy away and thwarts Miss Hardbroom with a magic restricting brooch. She also demands the Great Wizard hands over his title or she will not reveal the secret word (the name of her first familiar) that will release the girls, Miss Drill and Julie from Vanishment. Julie organises tower building with junk in the vanishment plane, and they all escape apart from Julie who cannot reach up to the vanishment hole, with only moments left when it starts to recycle.

Mildred and Maud search for the magic reversal word (the name of Agatha Cackle's first familiar) to reverse the vanishment and in the process discover Mr. Daisy locked away. He agrees to use his potion machine, so while Mildred and Maud delay Agatha with distractions and Ethel grabs the remains of Miss Cackle from Agatha, his machine's potion restores Miss Cackle. She in turn saves Julie with the secret word. Agatha banishes herself whilst Vanishment is still recycling, not wanting to return to the photograph.

As Mildred and Maud make their way to the end-of-term ceremony, where Mildred will be promoted to Head Girl, Miss Hardbroom tells Mildred the reason why she had been hard on her throughout her four years is because she is proud of Mildred for being herself. When they reach the assembly hall, Mildred learns that Izzy Jones is her sister and Spike is her dad, who Julie met in Morocco just before she was born.

As the end-of-term ceremony gets underway, Miss Cackle announces that she is back to where she belongs. Wormwood School is closed, thanks to Mildred and Maud's rebellion from the previous episode, which they are both praised for by the other girls. Cackles Academy is back to normal, and Mildred is promoted to Head Girl or valedictorian, in recognition for saving the academy once again, which delights everyone, including Ethel, who is relieved of the outcome.

Mildred accepts the position and expresses to the rest of the students, through an emotional acceptance speech, how far she has come since joining Cackles, comes to realization that magic is real as long as you believe in it, witch or not, and is grateful to her friends and family for their support.

As the school cheers for Mildred and celebrates the defeat of Agatha Cackle and the end of another term, Mildred, Maud, Azura, and Izzy joyfully embrace and Ethel reconciles with Sybil and Felicity.

After the credits roll, in a post-credit scene, outside the school during a party to celebrate Mildred's promotion, Ethel tells Mildred she'll do great as Head Girl, just before Mildred offers her to be her advisor, to which Ethel accepts. As Mildred leaves to join the celebration, the series ends with Ethel smiling in relief knowing that Mildred is not bad after all.