At Heathrow Airport, Lena Malley, Olga Fedori, a Polish immigrant working as a cleaner, starts a night shift that pairs her with her colleague Birdie Ainsley Howard. The two talk openly about their lives: Lena laments that she lives alone and has little contact with her family, while Birdie shares that she has a complicated past but is trying to move forward. Birdie introduces Lena to her adopted brother Elbie Toby Alexander, who also works at the airport and is mute. Lena explains that she doesn’t get on with her family, and Birdie notes scars on Birdie’s own arm as a reminder of her history. As the shift ends and Lena misses her bus, Birdie suggests a ride home, offering that her dad could drive Lena if they walk to Birdie’s house.
The moment they arrive at Birdie’s home marks the turning point: Lena is suddenly knocked out from behind and injected. She awakens to find herself dressed as a child and tied to a chair, with screams echoing from another room. Mum Dido Miles and Dad Perry Benson step in, presenting themselves as the authoritative heads of a disturbing household. Mum reassures Lena that cooperation will keep her safe, while Lena remains unable to speak due to the sedative. A subsequent injection deepens her sleep, and when she next wakes, she is subjected to brutal, ritualistic markings on her back. Mum forbids her to speak, and another injection sends her back under. She is finally restrained on a frame, and Dad lays out a chilling promise: Lena will become part of their family and must follow his rules inside his house.
Family dynamics quickly degrade into a steady drumbeat of humiliation and control. At breakfast, the family watches porn, and Lena’s escape attempts are thwarted when Dad grabs her and Elbie drags her back to the table. Birdie assigns Lena chores, and Lena’s attempts to escape are met with the dawning realization that her every move is watched. Lena finds and hides a mobile phone, only to be caught and subjected to heavier sedation. The atmosphere grows increasingly claustrophobic as Lena endures repeated punishments for no clear reason, and she becomes more vigilant about any sign of danger.
One night, Lena spots a man outside the house. When Dad intercepts, he drags Lena into the torture room, stores her in a suitcase, and orders Elbie to strike the case with a hammer. The intruder enters the home, and Mum and Birdie intervene, suffocating him with bubble wrap. The family dismembers the body and compel Lena to kiss the severed head, then cook the man’s remains as sausages. That night, Lena is bound to the kitchen radiator as further punishment.
The next phase of confinement sees Lena break free from the bedroom restraints and move through a house that feels almost like a maze. She discovers a staircase to an attic and encounters a mentally ill girl in a bed who struggles as Lena tries to retreat. In another room, a restrained man fights for his life as Lena finds a weapon. Christmas morning arrives with a hollow sense of ritual as Birdie notices the day’s tinsel and plastic cheer. Lena discovers the man from the attic crucified on a wall and the mentally ill girl seated in a chair, a grim symbol of the family’s perverse logic. Dad explains the birth tragedy that his daughter suffered, describing her as a “spastic” resulting from a cord around the neck, a rationale he uses to justify the cruelty in their home. In a cruel exchange, the family opens a bundle of inappropriate Christmas presents, and Mum confides that she forgot to purchase a present for Dad, implying that Lena could be his Christmas gift.
What follows is a brutal confrontation in Lena’s room. Dad arrives wearing a dress and makeup, presenting himself as “Mum” and making a crude sexual advance. Earlier, Elbie had loosened Lena’s handcuffs, allowing her to seize a weapon from the attic and defend herself. Lena stabs Dad and flees downstairs, with Mum and Birdie converging on her. In a desperate struggle, Lena stabs Mum and Birdie, driving them back toward the walls. The back door opens, and a tense moment unfolds as Birdie attacks Mum again, prompting Mum to strike Birdie with an iron and push her outside. Lena escapes through the back gate, but her ankle gives way, and she collapses in a field as Mum and Dad stagger after her. In a final, brutal sequence, Lena fights back and stabs Mum and Dad repeatedly, while Elbie releases the spastic girl and then strangulates her before walking away. The film closes on Lena screaming in the open field as the sound of planes taking off fills the air.
