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Insidious: The Last Key (also known as Insidious: Chapter 4) 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 terms of the series's in-story chronology. It stars Lin Shaye, Angus Sampson, Leigh Whannell, Spencer Locke, Caitlin Gerard, and Bruce Davison. It follows parapsychologist Elise Rainier as she investigates a haunting in her childhood home.

The film was released in the United States on January 5, 2018, by Universal Pictures. It has grossed $ 96 million worldwide and received mixed reviews from critics, who praised Shaye's performance but stated that the franchise had run its course.

Plot

In 1953, young Elise Rainier lives with her mother Audrey, brother Christian, and father Gerald in New Mexico. Gerald works as an executioner in a nearby prison. Elise claims to be seeing the ghosts of the prisoners that have been killed in the electric chair which concerns Audrey and angers Gerald. One night when Christian and Elise go to sleep, Elise encounters a ghost in their room which frightens the children. The commotion causes Audrey and Gerald to enter the bedroom and a furious Gerald beats Elise before locking her in the basement. In the basement, a voice lures Elise to a door which she unlocks. A demon called Key Face emerges from the door and possesses Elise. Audrey investigates and finds the possessed Elise before being hung by a wire controlled by the entity, killing her. Elise is broken out of the possession as Gerald arrives and finds Elise and Audrey's body

Decades later, an older Elise works as a paranormal investigator with her two colleagues, Specs and Tucker, taking clients via phone calls. She is called for help by Ted Garza, who has been experiencing paranormal activity since moving into his house. While on the phone he reveals that he is living in her childhood home. Hesitant at first, she explains to her colleagues what happened to her during her childhood, then deciding to accept the job in order to make things right.

While in New Mexico, Elise stays in her old home with Garza. He tells her that there have been a lot of paranormal incidents happening in the home, mostly in her old bedroom, which he has blockaded and refuses to enter. Elise does a walk through the home that night, using a night vision camera so the others can see what she does. While in her old room she stumbles across a whistle that Christian would use as a child to communicate to Audrey if he got scared. She then also finds a set of keys in her room attached to her father's old uniform and is led to the basement by mysterious sounds where a female ghost appears only visible to the crew. Elise is attacked by the ghost who tells Elise to "help her" and steals the whistle before vanishing. Elise then informs them that she had seen the ghost before in her teenage years. Elise had spotted the ghost and when Gerald investigated he claims to have not seen it. Fearing another beating by Gerald who is increasingly angry with Elise's abilities, she flees the house and abandons Christian.

The next morning, Elise, Tucker and Specs visit a diner where they encounter two sisters Melissa and Imogen. Christian enters shortly after and it is revealed that Melissa and Imogen are Christian's daughters and Elise's nieces. Christian is still furious at Elise for abandoning him with Gerald and storms out. Hoping to repair her relationship with her brother, Elise hands Melissa a photo of the whistle that she found last night and tells her to show Christian.

That night, while exploring the house, Elise and Tucker hear the whistle being blown from downstairs. The whistle leads Elise to the basement where she realizes that it is coming from behind a wall. Elise asks a series of questions and it is revealed to Elise that she will find what she is looking for behind the wall. Elise realizes that the wall is a door and rushes to her bedroom to get the keys. When Elise opens the door, she finds a missing girl who is chained to the wall. Garza storms into the room and it is revealed that he had been keeping the girl in his home against her will. He locks the group in the room and proceeds to try and kill Specs but Specs manages to trick Garza and kill him with a bookcase. The police rescue the group and Elise is taken in for questioning.

While being questioned by the police Elise is startled by a ghostly vision of Melissa in the police station and realizes that she is in danger. At the house, Christian, Melissa and Imogen enter to find the whistle. While exploring the house, Melissa is lured into the basement and attacked by Key Face who proceeds to stick two keys in Melissa; one robbing her of her voice and one that takes her soul which sends her into a coma. Elise, Tucker and Specs arrive and find Melissa in the basement. Christian demands that Elise stays away and leaves to the hospital with Melissa. Imogen stays behind, revealing to Elise that she can also astral project into The Further. Elise tells Imogen that tonight she intends to finally defeat Key Face to save Melissa and end the curse that is plaguing the house.

Elise and Tucker go in the basement to look for the ghost and search for the red door to enter The Further to rescue Melissa. Meanwhile Imogen and Specs try to find a Bible that Garza held dear, as Elise believes it can help them. While in the basement Elise sees something in the exhaust vent of the plumbing systems and finds a night gown that belonged to the female ghost, revealing her name to be Anna. A flashback reveals that the day Elise saw Anna in the laundry room she was alive and had escaped from the cellar room where Gerald kept her, similar to Garza. When Gerald walks in the laundry room he did see Anna, but pretended that there was nothing there. Elise also sees that after her younger self had run out of the house her father brutally kills Anna. Entering the vents, Elise discovers several suitcases filled with human remains of the various women that have been abducted by the men of the house. As Elise examines a suitcase, she is ambushed by Key Face who takes her into The Further and leaves Elise's body in the real world unconscious.

With Elise now in The Further and captured by Key Face, Imogen enters The Further and is led by Anna to a prison realm where key Face is holding the souls of those he has taken including Elise, Gerald and Melissa. In her cell, Elise realises that Key Face had been manipulating both Gerald and Garza feeding on the fear and hate generated by the abduction of the various women such as Anna and storing souls in the prison realm. Key Face tries to make Elise hurt Gerald much like he did to her as a child but Elise sees through the ruse and refuses to feed Key Face any more hate. Key Face attacks but Gerald saves Elise before being stabbed by Key Face causing his soul to vanish. Key Face captures Imogen and Melissa and chains them.

Key Face then stabs Melissa, causing her body in the real-world to start dying, and then hits Elise moving to silence her voice and take over her. Imogen then throws her the whistle. She uses it, prompting Audrey to come to her aid and save them, vanquishing Key Face. She reunites with her mother and tries to apologize for her death, but her mother has already forgiven her. Realizing Melissa is dying, the trio move to leave The Further quickly. Melissa's spirit returns to her body, saving her life. Elise and Imogen return to the real world and quickly head to the hospital to reunite with Melissa and Christian. Christian forgives Elise and she gives him the whistle, they hug and Elise leaves for her home in California.

In her sleep, Elise sees a vision of Dalton and the Lipstick Demon looming over him which frightens her awake. At that moment, her phone rings. The woman on the line explains that Elise helped her son a long time ago, and that now his son, Dalton, is experiencing the same symptoms. Elise agrees to help, saying she will meet him in the morning, setting up the events of the first film.