The setting of the film takes place in an area of West London, in 2002. At a local school, student Katie is seen to suffer intense physical and emotional bullying by a group of girls, as well as by Sam Peel (Noel Clarke). When her father picks her up from school that day, Sam quietly threatens to kill her if she ever tells anyone. That evening, Katie's older brother breaks into her room to discover that she has hanged herself.
The following morning, the students are informed of Katie's death and are given the day off to mourn. Trife and his best friends Jay (Adam Deacon) and Moony (Femi Oyeniran) decide to spend it smoking weed and drinking alcohol. Alisa, Trife's ex-girlfriend and also pregnant - supposedly with Sam's baby - decides to spend the day with her best friend Becky (Jaime Winstone). Becky performs oral sex on an older man in return for drugs, and aggressively coaxes Alisa into doing the same. The boys make their way to Sam's house on an estate to retrieve a Game Boy Sam had stolen off of them earlier. Realising Sam is out, the boys also steal Sam's cannabis and Jay has sex with Sam's girlfriend Claire (Madeleine Fairley). Sam returns unexpectedly, but is beaten unconscious by the boys and knock down Sam's mother as they flee.
Alisa and Becky unexpectedly run into some of Katie's bullies aboard a train. Alisa, feeling bad that she was not there for Katie, berates the girls for the suffering they caused. Becky accidentally reveals that Alisa is pregnant, information that the bullies promise to spread around school in an effort to humiliate Alisa. At the next station, Alisa hurries off the train to vomit, whilst Becky scorns her for putting her life at risk. Having successfully sold the drugs they acquired earlier, they head to a shopping centre to buy dresses for a party later that evening. Becky makes demands of Alisa to buy a cheaper dress, before the pair meet up with the boys. Jay, convinced that Alisa's baby is Sam's, informs her that Trife wants nothing to do with her (although he does still care about her). Heartbroken, Alisa asks Becky if they can leave, however Becky refuses as she wants to go to the party.
At the same time, Trevor has gone to see his Uncle Curtis (Cornell John). Curtis presents Trevor with a revolver, the same one Trevor had drilled the barrel for in the beginning of the film at school. Trevor is shown downstairs, where Andreas (a customer seen earlier to of missed a drugs payment) is tied and beaten by Curtis and Trevor. Curtis then orders Trevor to carve a "C" into Andreas' face with a Stanley knife in order to test him. Though visibly terrified, Trevor carries out his uncle's order, and flees the house traumatised. Trevor desperately tries to call Alisa, but is unsuccessful in doing so. On her way home, Alisa runs into a classmate, and persuades her to go to the party with him. At the party, Becky is stood up by Moony and fails in an attempt to convince Jay to have sex with her.
Trevor interrupts Alisa and the other classmate who are kissing outside, and confesses his love for her. Alisa informs Trevor that the baby is definitely his - she had never slept with Sam. The two rekindle their love, but a vengeful Sam arrives at the party and attacks Trevor. Alisa hurriedly tells Jay and Moony, who intervene to help Trevor. Outside, Sam beats down both Trevor and Jay, whilst intimidating Moony into not interfering. Sam challenges all the other party goers who come out to watch, however Alisa, the only one unafraid of Sam challenges, slaps him. When Sam grabs her by her hair, Trevor gets to his feet and fights him to the ground. Alisa pleads with him to stop, and he ambles over to her. However, Sam takes this opportunity to grab his baseball bat, and deliver a critical blow to Trevor's stomach. As this occurs, a car pulls up to the party. Brandishing a gun, the assailant demands to know who Sam is - a friend of Katie's points out Sam to him when he pretends he isn't. Removing his scarf, the assailant is shown to be Lenny, Katie's older brother. Lenny forces Sam to his knees at gunpoint, and produces the note Katie wrote before she hung herself. Lenny orders Sam to tell him why he shouldn't kill him. With his dying breath, Trevor shouts "because he isn't worth it". Sam is almost killed when he insults Lenny after the latter begins to walk away, however the gun fails to fire. Sirens are heard in the distance, so Lenny and as his accomplice flee. Trevor dies before the ambulance and police arrive.Aml Ameen as Trevor
Red Madrell as Alisa
Adam Deacon as Jay
Noel Clarke as Sam Peel
Jaime Winstone as Becky
Femi Oyeniran as Moony
Madeleine Fairley as Claire
Cornell John as Curtis
Rafe Spall as Lenny
Nicholas Hoult as Blake
Rebecca Martin as Katie
James Witherspoon as Kilpo
Ortis Deley as Derek
Stephanie Di Rubbo as Shaneek
Kate Magowan as Stella
London hip-hop group Arkane wrote the title track for the film. The film was principally shot in the actual areas in which it is set around London W11; for example, some of the school scenes are shot in Twyford CE High School in Acton, similarly Alisa and Becky's journey on the London Underground is between Ladbroke Grove and Royal Oak stations.
Kidulthood has received a generally positive critical response. Writing in The Guardian, Miranda Sawyer called the film "a rollicking UK youth ride, cinematically filmed, persuasively acted and bumped along by a fantastic all-British soundtrack... It's also very funny, laced with a humour of the slapped-in-the-face-with-a-kipper sort: you can't help laughing because it's so outrageous". Stephen Armstrong in The Times, however, said "the only people who should be shocked by this film are people who have never been teenagers. What Kidulthood does is take all the violence, sex and intoxication experienced in a teenage year and condense it into a single day, because that's far more marketable than a film about eight kids spending four hours sitting on the swings wondering what to do". The Daily Mirror described it as being "as potent as a shot of vodka before breakfast – a harrowing, uncompromisingly bleak but thoughtful look at the anguish of being young and poor in Britain".
The film spawned a trilogy, receiving two sequels: Adulthood was released in 2008, which was written and also directed by Noel Clarke, and then Brotherhood in 2016.