In 1953, young Elise Rainier lives with her parents Audrey and Gerald, a prison executioner, and her brother Christian, in New Mexico. Elise claims to see the ghosts of the prisoners that have been executed, alarming her parents. One night, Elise encounters a ghost in their room. Frightened, Christian looks for the whistle their mother gave them to call for help, but it’s lost. Gerald, furious, beats Elise and locks her in the basement. A demon called “Key Face” comes through, possessing Elise and killing Audrey by hanging her. Gerald arrives to find Elise and Audrey’s body.
Decades later, Elise works as a paranormal investigator with her colleagues, Specs and Tucker. Ted Garza calls Elise, saying he’d been experiencing paranormal activity at his house, which is actually Elise’s childhood home. She hesitates, but accepts the job.
Ted explains that many paranormal incidents have happened, mostly in her old bedroom. Elise finds the lost whistle in her room. In the basement, she is attacked by a female ghost that shouts “Help her!” before vanishing with the whistle. Elise tells them that she has seen her before, as a teenager, but when her father said he saw nothing, Elise fled the house in fear of another beating, abandoning Christian. The next morning, Elise, Tucker, and Specs meet sisters 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.
That night, Elise and Tucker hear the whistle being blown. The sound comes from behind a wall that is actually a door; behind the door is a bloodied girl with a chain around her neck. Ted storms into the room and it’s revealed that he was keeping the girl against her will. He locks the group in and tries to kill Specs, but Specs kills him with a bookcase. The police rescue the group.
Christian and his girls go to find the whistle and Melissa is attacked by “Key Face”. The entity sticks two keys in Melissa: one steals her voice, and the other takes her soul and sends her into a coma. Christian demands that Elise stay away and Melissa is taken to the hospital.
In the basement, Elise and Tucker find the red door that is the entrance into The Further. Elise also finds a night gown that belonged to the female ghost, Anna. A flashback shows that Elise had seen Anna in the laundry room. Anna was actually alive and had escaped from the room she was locked in. When Gerald walked in, he saw Anna, but pretended he saw nothing. After Elise ran away, Gerald brutally killed Anna. Inside the vents, Elise finds several suitcases filled with the remains of women that had been locked in the house. She is then ambushed by Key Face who takes her into The Further, leaving her unconscious body in the real world.
Imogen enters The Further, led by Anna into a prison realm where Key Face is holding all of the souls he has taken. Elise figures Key Face had been manipulating both Gerald and Ted by feeding on the fear and hatred generated by the women, whose souls were trapped in there. Key Face tries to coerce Elise into hurting her father as revenge for what he’s done. Elise sees through the ruse and refuses to feed Key Face any more hatred. Key Face attacks Elise, but Gerald saves her before he is stabbed by Key Face, and his soul vanishes.
Key Face stabs Melissa, causing her physical body to start dying. He attempts to possess Elise. Elise blows the whistle, and Audrey saves them, vanquishing Key Face. With Melissa dying, they move to find her body in The Further. They open a door and see a young boy, Dalton Lambert, falling from a ladder, and he sees them. Realizing they opened the wrong door, they leave and find Melissa in the next door. Melissa’s spirit returns to her body in the real world, saving her life. Elise makes amends with her mother and says goodbye. Elise and Imogen return to the real world and reunite with Melissa and Christian. Christian forgives Elise and she gives him the whistle.
In her sleep, Elise sees a vision of Dalton, with the Red-faced demon looming over him. A woman, Lorraine, calls for help for her grandson, Dalton. Elise had helped her with her son Josh years earlier and she agrees to help Dalton.