Insidious: The Last Key is a 2018 American supernatural horror film directed by Adam Robitel and written by Leigh Whannell. It is produced by Jason Blum, Oren Peli, and James Wan. It is the fourth installment in the Insidious franchise, and the second in the chronology of the story running through the series. Starring Lin Shaye, Angus Sampson, Whannell, Spencer Locke, Caitlin Gerard, and Bruce Davison, the film follows parapsychologist Elise Rainier as she investigates a haunting in her childhood home.

Talks for a fourth installment in the franchise began in June 2015, with Whannell saying the next film would take place shortly before the first film. In May 2016, it was announced that Chapter 4 would have an October 2017 release date with Whannell writing, Blum, Peli and Wan producing, Robitel directing, and Shaye returning to reprise her role as Elise Rainier. Principal photography began in August 2016, and ended the following month.

The film was released in the United States on January 5, 2018, by Universal Pictures. It grossed $167 million worldwide and received negative reviews.^[3] Insidious: The Red Door was released on July 7, 2023, continuing the storyline of the first two films.

In 1953, Elise Rainier lives in Five Keys, New Mexico, with her parents, Audrey and Gerald, and younger brother, Christian. Elise and Christian encounter a ghost in their bedroom. Frightened, Christian looks for a whistle their mother gave him to call for help, but can't find it. Gerald, furious, canes Elise and locks her in the basement. Elise opens a mysterious red doorway and is briefly possessed by a demonic spirit. Audrey is killed by the demon.

Decades later in California in 2010, Elise works as a paranormal investigator with her colleagues, Specs and Tucker. A man named Ted Garza calls, saying he has been experiencing paranormal activity at his house. Realizing it's her childhood home, Elise departs to help him. While investigating, she finds Christian's lost whistle, but it disappears again after she encounters a female spirit. Elise tells Specs and Tucker that she had seen the spirit before when she was a teenager. She had fled the house in fear of another beating from her father, abandoning Christian.

Elise, Tucker, and Specs meet Melissa and Imogen, Christian's daughters. Christian is still furious at Elise for abandoning him. Hoping to repair their relationship, Elise hands Melissa a photo of the whistle, telling her to show it to Christian. Elise and Tucker discover a hidden room in the basement. Guided by the female spirit, they discover a young woman being held prisoner inside. Ted reveals that he is responsible. He locks the group in and tries to kill Specs, but Specs kills Ted in self-defense.

After police clear the house, Christian and his daughters go inside to find the whistle. Melissa is attacked by the demon from Elise's past, known as Keyface. Keyface sends her into a coma with her consciousness now stuck in the spirit realm of the Further.

Trying to save Melissa, Elise searches the house and discovers hidden suitcases containing belongings of numerous other women who had been held prisoner, including the young woman she had seen as a girl. Elise realizes that like Ted, her father, Gerald, had also kidnapped women and held them in the secret room. The woman she saw as a girl, Anna, was actually alive then, not a ghost; she had escaped the hidden room and found a passage leading to the laundry room. Gerald saw Anna hiding when he confronted Elise about seeing someone and lied to her, precipitating her leaving the home. Elise temporarily stunned Gerald by grabbing his face and showing him the manner of his death, giving her time to escape. Once Elise ran from him, Gerald went back and killed Anna. In the present, Elise is ambushed by Keyface and her spirit taken into the Further.

Adult Elise encounters her younger self in the Further, where she explains to adult Elise that she knows her daddy is just scared when he punishes her and that Keyface wants her to help him open all the doors in the Further. Imogen, who possesses abilities like Elise's, enters the Further and is led by Anna's ghost into a prison realm where Keyface is holding all of the souls he has taken, including Melissa and Elise. Elise realizes Keyface had been controlling both Gerald and Ted and feeds on the fear and hatred generated by the women they kidnapped. Keyface tries to coerce Elise into hurting her father's spirit as revenge for what he has done. Elise starts beating Gerald, but is stopped by Imogen and refuses to feed Keyface any more hatred. Keyface captures Imogen's soul and then attacks Elise, but Gerald tries to save her before he is stabbed by Keyface, his spirit vanishing.

Keyface stabs Melissa, causing her physical body to convulse, then flatline. He attempts to possess Elise but Elise blows Christian's whistle and Audrey's spirit arrives, vanquishing Keyface. Audrey releases Elise, Melissa and Imogen's chains from their spirits and they search for their bodies on the way out. While searching, they open a door and see a young boy, Dalton Lambert, fall from the ladder in his attic. Realizing they opened the wrong door, they leave the door open and find Melissa's body. Melissa's spirit returns to her body in the real world, saving her life. Elise makes amends with her mother's spirit and she and Imogen return to their own bodies and reunite with Christian who forgives Elise.

In her sleep, Elise has a dream about Dalton and a red-faced demon. She awakens and receives a call from Lorraine. Elise had helped her son years earlier and now her grandson, Dalton, needs the same help, which Elise agrees to provide.

Insidious: The Last Key was released on January 5, 2018.^[9] The film was then released a week later on January 12, 2018, in the United Kingdom.

On October 29, 2020, it was announced that a sequel was in the works with Scott Teems writing the screenplay based on a story by Leigh Whannell and series star Patrick Wilson directing. It focuses on a grown-up Dalton, a role reprised by series star Ty Simpkins, as he heads off to college.^[19] It released on July 7, 2023.