The film opens showing a school at lunch break with children playing football, while middle class student Blake (Nicholas Hoult) gives out invites to a party. The scene switches to Trevor (Aml Ameen) using the drill press, boring out an unseen object. Alisa (Red Madrell) comments on how she "doesn't feel well" and thinking about not attending the party, but her best friend Becky (Jaime Winstone) encourages her to go. We see Sam (Noel Clarke) spitting into Katie's hair and asking her where his girlfriend Claire (Madeleine Fairley) is, but Katie says she doesn't know. Jay (Adam Deacon) is seen kissing Claire; she asks Jay if he is scared and he replies negatively.

The film then uses the "day in the life of" device, beginning with a group of "kidults" getting the day off school after Katie's suicide as a result of being bullied. The film then slowly builds up to the climactic house party. It revolves around three teenagers: Trevor, who is more commonly known by his street name Trife, Jay and Moony (Femi Oyeniran).

Trife is being tempted into the gangster lifestyle by his uncle who requests him to do illegal errands, but simultaneously Alisa is offering a chance to a better life. However, a rumour that Alisa has slept with someone else might influence this life-changing decision. Trife also has to deal with the school bully, Sam, who is out for revenge after Jay steals his girlfriend Claire, and after Trife, Jay and Moony beat Sam up in his own house during a break-in to retrieve Mooney's sister's Game Boy. On their escape from Sam's house, they push Sam's mother which further enrages him.

At the same time, Alisa has just learned that she’s pregnant and her friend Becky wants to take her out on a drug and shopping binge. The film heads toward a conclusion with Katie's brother set on revenge for his sister’s suicide and with Sam looking for payback at the house party thrown by Blake. Trife and Alisa reconcile, while Becky tries to hook up first with Moony and then with Jay. Sam arrives, armed with a baseball bat, and Trife is seriously injured after a blow to the stomach. Katie's brother Lenny arrives, brandishing a pistol he procured from Trave's uncle. He threatens Sam, who cowers and cries in fright, but Trife prevents Sam's murder with his final words of "He's not worth it". Then, as Katie's brother is leaving, Sam insults him. Lenny fires the pistol, only to have it misfire and explode. Lenny and Sam flee the scene as the paramedics arrive to help Trife, but Trife's injury is fatal and he dies.