Liz Dunn resurfaces, a private school student, disheveled and bloodied after vanishing 18 days earlier with her friends Mike, Geoff, and Frankie. She is interviewed by a psychiatrist, Dr. Philippa Horwood. Liz recounts how her friend Martin arranged for the four to spend the weekend in an abandoned underground nuclear fallout shelter to avoid a school field trip. Liz portrays herself as unpopular, but Frankie is her friend, and together Frances 'Frankie' Almond Smith convinces the others to go down into the shelter. When Martin fails to return for them, the four realize they are trapped and begin to turn on one another. They soon discover hidden microphones in the shelter placed there by Martin. Attempting to get Martin's attention, Frankie pretends to be ill, while Mike and Liz feign hatred for one another; Martin has had unrequited romantic feelings for her since their childhood. Liz claims they woke up one morning and found the hatch opened, allowing them all to finally escape.
The doctor is skeptical of Liz's story. Martin is subsequently taken into police custody, where he tells an entirely different story: Liz and Frankie orchestrated the scheme to get Liz to know Mike better, and for Frankie to spend time with Geoff. Liz is not the unpopular loner she has portrayed herself as: in fact, it is Martin who is the loner while Liz and Frankie are among the popular girls. Meanwhile, Liz returns home, where she experiences disturbing flashbacks about what happened. An enraged Martin goes to visit Liz, believing she is framing him. She runs from him through the garden and approaches a weir. Martin cries, and Liz says she knew they would let him go because they could not prove anything.
At their next meeting, Liz tells Phillip[a] that she cannot remember what happened in the shelter. Despite the doctor's initial protestations, she agrees to Liz's request to go back to the bunker, hoping to invoke her memory. Once inside, Liz reveals the truth: she had locked herself and her friends inside in the hopes of winning the affection of Mike, with whom she is obsessed. After discovering that both he and Geoff had slept with Frankie, she spontaneously decided to lock the door, isolating them and allowing her to become closer to Mike. The four had planned to drink and do drugs in the shelter, but the realisation that they are unable to escape throws the group into hysteria. Frankie soon becomes ill and is unable to stop vomiting. This increases her dehydration, tears the lining of her stomach, and puts a great strain on her heart (which is weakened by bulimia) leading to cardiac arrest and death.
Liz, Mike, and Geoff gradually run out of food and water. When Mike discovers that Geoff is hoarding a Coca-Cola in his backpack, he attacks him in a fit of rage and incidentally kills him by beating Geoff's head repeatedly on the floor. Liz suggests a suicide pact whereupon Mike professes his love for her; this prompts her to climb the ladder to the shelter's entrance and unlock the door. When Mike discovers that she had the key all along he attempts to chase after her, rapidly climbing the ladder and causing it to break. Mike falls, is impaled by the broken ladder, and dies.
After Liz finishes recounting the story, Phillip[a] asks her to make an official statement corroborating Martin's version of events. Liz refuses, having murdered Martin when he visited her the day before. Police arrive at the shelter and Liz begins screaming for help, pretending Phillip[a] is attempting to hurt her. Meanwhile, Martin's corpse is fished out of the river, and in his pocket, the police find the key to the shelter which implicates him in the events. The police attribute his death to suicide. Liz is allowed to go free. She leaves the shelter in the back of an ambulance, smirking at Phillip[a] from the window as it drives away.
