At an end-of-term Halloween house party, Tom Kane as Felix arrives alone, drawing a quiet line through a room full of loud friends. The group includes Jack Gordon as Chris, a spoiled brat, paired with his mild-mannered girlfriend, Florence Hall as Gemma, his friend Liam Boyle as Paul with his own girlfriend, the abrasive Jennie Jacques as Eleanor, and his drug-dealer friend, Alexander Vlahos as Luke. Felix watches Gemma from the shadows for most of the night, and Luke’s attention lingers on him too. Luke offers Felix a line of cocaine and nudges him toward Gemma, aware of what those choices might unleash, while Gemma politely refuses Felix’s advance.
Meanwhile, Paul and Eleanor attempt to find a moment for themselves in an upstairs room, but Paul passes out drunk. Disappointed but not defeated, Eleanor convinces the others to play truth or dare downstairs. The game spins to Felix, who chooses truth, and Eleanor asks him to name the person he would most like to fulfill a private fantasy with. Reluctantly, he names Gemma, who seems a little embarrassed by the attention. Chris grows jealous and lashes out, punching Felix, and Gemma ends up comforting the beleaguered boy she’s chosen, while Chris’s temper cools only briefly before seething again.
Months pass, and after the Christmas holidays the five friends receive invitations to a surprise birthday party for Felix. They are lured to Felix’s mansion, only to discover that the “mansion” is really a remote, seemingly abandoned property with a separate outbuilding. A bounds-wary groundsman, Woodbridge, directs them to a rundown shack half a mile down the forest track, where they meet Justin, Felix’s elder brother. Justin is a polished veteran, recently back from three Afghan tours, and he immediately reveals a harsher, more controlling side—quick to pepper his speech with homophobic slurs and a chilling sense of entitlement.
Justin explains that Felix will not be joining them due to a last-minute flight cancellation, but he welcomes the guests to stay for drinks. The atmosphere grows tense as Justin escalates the party mood with a twisted game of truth or dare, and he quickly discloses the truth about Felix’s death in a calculated way: the young man hanged himself from the shack’s roof the night after the Halloween party, and a postcard in his pocket read, “Truth or Dare, bitch!” The revelation hits with brutal force, and Justin demands to know who wrote the postcard, promising that the only way to reconcile the family’s shame is to bring the culprit to justice.
When the group attempts to leave, Justin shoots Paul in the leg, and coerces Luke into acting as his partner in a grim night of coercion, binding the others. He unveils a torture game he dubs “The Acid Test,” forcing Gemma to participate in a terrifying, life-threatening dare. A tube is fitted with two jars—one said to contain water, the other battery acid—and Chris is pressured into choosing first. He selects the jar believed to contain water, and the game proceeds. Then the roles are inverted, and Paul is coerced into choosing a jar; the swapped jars leave Paul facing the acid, and Chris dies, bleeding from his mouth.
Their ordeal is interrupted by an uninvited intruder: Jonesy, Luke’s drug-dealer friend. The standoff escalates quickly as Justin breaks Jonesy’s arm and forces Luke to confront a grim choice: shoot Jonesy or shoot Justin, with the danger of either outcome sealing their fate. Luke chooses to fire, and Jonesy dies. Gemma, seizing an escape opportunity, slips free through a back window.
Gemma makes for the mansion, hoping to find help, only to confront a startling sight: Felix is alive but bedridden. Woodbridge arrives with a shotgun and admits he found Felix hanging and has kept him alive ever since, a truth that ignites a brutal struggle that fatally wounds Woodbridge. Justin arrives at the mansion, and in front of Felix, shreds Gemma’s body with a brutal act, breaking her neck.
Back at the shack, Luke is wary about releasing Paul and Eleanor, still fearing Justin’s reach. He confesses to having sent the postcard and to his broader role in the carnage. Eleanor, however, seizes the moment to seize control of the situation by turning the tide: she suddenly bites off Luke’s little finger in a brutal display, sending him backward where he impales himself on a nearby tool and dies. Paul and Eleanor finally break free, but their escape is thwarted by Paul’s bleeding leg, and they stumble toward possible safety.
Justin returns with Felix in a wheelchair, and a violent confrontation erupts between the trio. Eleanor manages to shoot Justin in the shoulder and smashes a jar of battery acid over his head, badly burning him. The struggle ends with Justin eventually tied to a post as Paul tries to start his car for escape. Eleanor reveals the deeper truth: the postcard wasn’t the real trigger for Felix’s suicide. A video she had used to blackmail him—a recording of Felix performing oral sex on the unconscious Paul—was the real catalyst; the fear of his brother’s discovery drove Felix to the edge.
With the revelation out in the open, Eleanor slips away with Paul, leaving Felix’s mansion in flames behind them. In a final, devastating turn, she hands Felix a grenade and escapes as the house explodes, killing both Felix’s brother and Justin in the fiery collapse. The film closes on the ruins of the property, the truth laid bare, and the destructive reach of pride, secrets, and vengeance lingering in the ashes.
