A young orphan named Reiko offers herself as a sacrifice to a 5000-year-old dragon named Ravendia in exchange for protecting her village from the Demon Lord. However, Ravendia is actually a cowardly dragon who prefers a vegetarian diet. When Reiko insists he eats her or she kills herself in disgrace, he pretends to have eaten her soul as a sacrifice, causing Reiko to believe she is now his disciple. Returning Reiko to her village, Ravendia hopes he can return to his cave for a few hundred years until the village forgets his existence, but Reiko’s friend, Raiotto, is furious that he ate Reiko's soul and tries to attack him. Suddenly, Dark Wolves attack the village, and Reiko reveals her belief in the loss of her soul has unlocked powerful repressed magical abilities and she drives the wolves away. Based on his 5000 years of experience, Ravendia realises Reiko might just be the most terrifyingly powerful wizard in history.

Despite Raiotto’s promise to save Reiko’s soul, Reiko believes she and Ravendia should leave the village to begin hunting the Demon Lord. Reiko steals a magician’s outfit from Raiotto’s home, though he had intended it to be a gift to her anyway, and departs with Ravendia. To avoid the Demon Lord, Ravendia claims that he must discover and join whatever rebel army who is currently opposes the Demon Lord. Reiko uses her clairvoyance to direct them to Paleodona City where strong adventurers gather and Ravendia hopes he can abandon Reiko there and quietly slip away. Unfortunately, Paleodona is under attack by monsters whom Reiko defeats. With his 5000 years of experience, Ravendia is able to direct Paleodona’s General, Aliante, to the monster who is controlling all the other monsters, thus saving the city. That night, Reiko has a nightmare about her father, but she is comforted by Ravendia’s presence.

Ravendia attempts to convince Aliante that Reiko’s power does not come from him. Aliante warns Ravendia that Reiko is still a novice at controlling her vast magic, so there is a chance that it will corrupt her and actually turns her into a true demonic dragon. She also asks if Ravendia really intends to oppose the Demon Lord, and when he admits he is unsure, she reveals she is a General of the Demon Lord's army and attacks him. Reiko awakens and protects Ravendia but is swiftly defeated as she has no experience with swords before. Aliante reveals her sword doesn’t cut; it just causes intense pain, and hits him repeatedly. Eventually, Aliante is forced to accept that Ravendia really is a weak coward, so she admits her loyalty to the Demon Lord was a lie to test him and agrees to keep his weakness a secret. Realising he has to stay with Reiko, Ravendia agrees to convince her that the fight with Aliante was a dream. The next morning, Reiko awakens and, to her incredulous shock, finds out that Ravendia has shrunk to the size of a large dog.

Ravendia is revealed to have shrunk due to a potion given by Aliante, since his large size was inconvenient to live in a city, though he lies and tells Reiko that he shrank himself. Raiotto arrives at Paleodona to search for Reiko but when he learns Aliante fought against Ravendia, he asks her to become her disciple, so she promises to put him through hellish training. Having left Paleodona, Ravendia is starving but cannot eat anything in case of Reiko discovers he is a vegetarian. They are stopped by the bandits who want to sell both her and Ravendia on the black market. Reiko easily beats up the bandit leader who apologises profusely and even agrees to free the slaves he has imprisoned in their secret cave. Ravendia notices a second cave nearby so he sends Reiko to escort the freed slaves and imprison the bandits. Left alone, Ravendia enters the second cave hoping to feast on his favourite cave moss, only to find out that the cave is a home to a dark spirit.

Ravendia attempts to flee but the dark spirit turns out to be friendly, and prepares Ravendia a vegetarian feast. The dark spirit explains he was once the God of Hunting who first taught the humans to hunt, but once the humans no longer needed him, he was slowly forgotten and has been alone for centuries. Feeling grateful for the feast and sympathetic for him, Ravendia asks him to learn how to hunt and the God of Hunting promises to teach Ravendia everything about hunting in only three hours. However, due to various factors, Ravendia fails every lesson, from physical fitness to archery, even to unarmed combat against a rabbit. Dejected at his failure, the God of Hunting grants Ravendia a spell to strengthen his claws, then returns to his cave thoroughly depressed with both promising never to mention their failed training to anyone. Ravendia falls asleep until Reiko returns and reports she has found another city called Selianen, which is large, prosperous and only protected by a shallow moat so she demands that they capture it for themselves before the Demon Lord does.

Approaching Selianen, Reiko realises the city is a home to the Water Saint. The Saint contacts Ravendia to threaten him but Reiko easily uses her own powers to scare the Saint away and deactivate Selianen’s magic barrier. Now able to enter Selianen, the citizens explain that many years ago, the Saint created an oasis from the desert, allowing Selianen to grow into a city. Ravendia and Reiko lodge at an inn with the former decides to search for the Saint to clear up the misunderstanding. The Saint is so scared that she hides in whatever water she can, but by coincidence, a thirsty Ravendia keeps trying to drink the water she is hiding in, so she keeps running away. The children notices Ravendia is starving and offers him a fruit, but the Saint thinks he means to eat the children and leaps to defend them. Thirsty and hungry, Ravendia just wants to go back to the inn. The Saint, thinking she is being blackmailed, agrees to guide him there if he spares Selianen. Ravendia agrees and reminds her that she needs to repair the barrier, which she completely forgot.

Reiko is unhappy that the humans worship the Saint as she knows that she is actually a Demon who can manipulate water. An evil dragon who is loyal to the Demon Lord and a demon named Xu senses that the barrier is weaken and decides to destroy the Saint. The Saint, poorly disguised as a maid, tries to urge Ravendia to leave, but when she serves him rotten food, Reiko attempts to kill her, causing the Saint to flee. Seeing the wasted food, Ravendia scolds Reiko for using her power recklessly when it should be used to help others, so he forbids her from using her powers except in self-defence before sending her to play with the children. Reiko is confused by the children’s description of Ravendia as “small and cute” while she sees him as “great and powerful” and becomes convinced they must mean that a second dragon is somewhere nearby. Ravendia is teleported underwater by the Saint in an attempt to drown him. The Evil Dragon attacks Selianen but Reiko, confusing him for the “small and cute” dragon, approaches him.

The Evil Dragon takes Reiko’s invitation to play as a dual challenge and accepts. Reiko, having misunderstood the children’s instructions to “play with the cute dragon”, throws the Evil Dragon like a football, repeatedly crashes him into the ground. The magical shockwaves from throwing the Evil Dragon into the ground causes the Saint to lose focus and her spell to drown Ravendia fails. Ravendia is saved when the strengthened claws he got from the God of Hunting becomes a Black Nebula that lifts him out of the ocean. The Saint constructs a water cage to drain Ravendia’s magic. This, however, drains the spell to keep Ravendia small. Thus, he explosively returns to his original giant size, sending him and the Saint back through the portal to Selianen. The Saint believes she has failed but Ravendia insists she earned the title of a Saint through her good deeds and deserves to stay in Selianen. The Saint finally believes that Ravendia is a good dragon. A Death Locust, a low rank demon that spoils the crops, steals Xu and infects the Saint with him.

The Saint reverts to an evil water demon, threatening to drown Selianen and turn it into a swamp. However, the mud she creates to flood the streets is mistaken by the villagers as fertile soil to help their farms, so they worship her even more. The Saint cries at her failure and tells Ravendia that when she was in the Demon Lord's army, she was bullied for being weak. To prove herself, she attempted to victimize the refugees who are lost in the desert, but her rain was hailed as a miracle. The refugees build farms and eventually, the city of Selianen where they worshipped her as a Saint. Ravendia suspects the Saint can no longer tell if she is a Saint or a demon. A little girl thanks the Saint for her blessings and Ravendia points out that a person’s past doesn’t matter as long as they keep trying to be better. The Saint returns to normal due to Xu leaving her but infects Ravendia, planning to expose his evil nature instead.

The Saint invites both Reiko and Ravendia to stay and attend the Saint’s Festival. Xu struggles to find any evil in Ravendia’s heart but is determined to find something. Meanwhile, the Evil Dragon himself wanders the fields outside Selianen, sulking as he is stuck between his desire to destroy Selianen and his fear of Reiko, where he is noticed by the adventurer’s guild. The news reaches Aliante, who stops Raiotto from leaving to save Reiko until his training is complete. She goes by herself instead to see what trouble Ravendia has caused. At the festival, Ravendia is starving but the sheep keeps stealing his vegetarian food, which is being given by the little girl, causing him to wish that the food was all his. His brief selfish thought is enough for Xu to urge him to give in to his selfishness. Ravendia attempts to fight the sheep for all the food, but they beat him up instead, returning him to normal. The little girl promises to find food for him and the Evil Dragon who is still sulking in the fields. This makes Ravendia learnt about the Evil Dragon.

Ravendia cannot believe over how psychologically damaged the Evil Dragon is after Reiko used him like a football. Xu finally returns and forcefully possesses the Evil Dragon in an attempt to possess Reiko so that he can rule the world, including Selianen. He attempts to kill Ravendia but Reiko saves him. Despite the Evil Dragon's warnings, Xu infects Reiko but finds out that the darkness in her heart is so powerful that he is sucked into it and devoured. This causes the memory of Reiko’s father being killed by the monsters to emerge. The sudden traumatic memories cause Reiko to unleash a massive magical explosion, destroying the countryside as she begins to painfully transform into a demonic dragon. Aliante arrives just in time as Reiko plans to kill the Saint for continually disrespecting Ravendia, showing her belief in Ravendia still exists despite her madness. Faced with such overwhelming powers, Aliante says they should run. Ravendia hopes this might be his chance to escape from Reiko as she is now so powerful that she couldn’t possibly need him anymore. However, he can't bring himself to abandon her and insists on staying.

Determined to protect someone else rather than himself, Ravendia approaches Reiko but is pushed back by her magic. Aliante realises due to Reiko still believing in Ravendia, her magic doesn’t actually hurt him, which she proves by using the unfortunate Ravendia as a shield. While Reiko continues to painfully transform into a demonic dragon, Ravendia decides to return to his original giant size to reach her. Aliante distracts Reiko while the Saint extracts the potion from his body that keeps Ravendia small. Now giant again, Ravendia uses the now unconsious Evil Dragon’s wings and his own Black Nedula claws to slingshot himself at Reiko. The impact makes Reiko remembers a comforting memory of her father, returning her to a human. Ravendia is relieved that Reiko is back to normal but still forces her to apologise for partially destroying Selianen. The Saint announces that the festival will still continue but is slightly jealous over how the people hail Aliante as a hero instead of her. Reiko and Ravendia volunteer to help with rebuilding Selianen with the latter tells the former that if she ever feels like she is losing control of her powers again, she should simply think of the precious memories of her father. Reiko promises that she will and smiles happily to him.