A wild, freeform, Rabelaisian trip through the darkest recesses of Edinburgh low-life, focusing on Mark Renton and his attempt to give up his heroin habit, and how the latter affects his relationship with family and friends: Sean Connery wannabe Sick Boy, dimbulb Spud, psycho Begbie, 14-year-old girlfriend Diane, and clean-cut athlete Tommy, who's never touched drugs but can't help being curious about them...
Renton, deeply immersed in the Edinburgh drug scene, tries to clean up and get out, despite the allure of the drugs and influence of friends.
- Set in Edinburgh, the film begins with Mark Renton's (Ewan McGregor) narration as he and his friend, Spud (Ewen Bremner) sprint down Princes Street, pursued by security guards. Renton states that unlike people who 'choose life' (traditional families, materialistic possessions and financial stability), he decided live life as a heroin addict. The rest of Renton's close circle of football enthusiast friends are introduced: con-artist Sick Boy (Jonny Lee Miller), clean-cut athlete Tommy (Kevin McKidd) and sociopath, Frank Begbie (Robert Carlyle). Sick Boy, Spud and Renton are all heroin addicts who spend their days shooting up in their drug dealer, Swanney's (Peter Mullan) flat. Swanney is often referred to as "The Mother Superior" by the guys.
One day, Renton decides to give up heroin. Realizing he needs one last high to get him through the night, he buys opium rectal suppositories from dealer, Mikey Forrester (Irvine Welsh). After this final hit (and a nasty spell of diarrhea), Renton locks him into a cheap hotel room to endure the withdrawal. He later goes to a nightclub with the gang, noticing that his sex drive has returned with a vengeance. He eventually leaves with a young woman named Diane (Kelly Macdonald). After having sex together, Diane refuses to sleep in the same room as Renton and forces him to sleep in the hallway on a sofa. In the morning, Renton realizes that Diane is in fact a 15 year old schoolgirl and the flatmates sitting around the breakfast table are in fact, her parents. Horrified, Renton tries to shake off the incident but remains friends with Diane, who threatens to tell the police that Renton had sex with her despite being underage.
Tommy is dumped by his girlfriend Lizzy (Pauline Lynch) after a string of events unintentionally initially started by Renton. Renton had previously borrowed a sex tape of Tommy & Lizzy, without their consent, hiding it in the box of a football tape. Lizzy angrily believes that Tommy had returned the tape to the video store. Sick Boy, Spud and Renton decide to use heroin again and a broken-hearted Tommy asks to try it to see why they like it so much, despite Renton's reluctance.
One day during a heroin-induced stupor, the gang are violently interrupted by Allison (a fellow heroin addict) who has discovered that her infant child, Dawn has died from neglect. All are shell-shocked, especially Sick Boy who is implied to be the baby's father.
Renton and Spud are later caught stealing from a book shop and are pursued by security guards, as seen in the beginning of the film. Due to prior convictions, Spud is sent to prison whereas Renton is put on a Drug Intervention Programme and given methadone. Despite support from friends and family, Renton becomes increasingly bored and depressed and escapes to Swanney's flat where he takes a particularly strong hit and nearly dies. Renton's parents lock him in his room so he can beat his addiction cold turkey. As Renton lies in bed going through severe withdrawal symptoms, he hallucinates that Diane is singing to him, friends are giving him advice and Allison's dead baby is crawling on the ceiling. The hallucination sequence is intercut with weird, imagined scenes from a TV game show, with Dale Winton asking Renton's parents questions about HIV. Renton is finally awoken from his nightmares and is told by his parents that he needs to get tested. Despite years of sharing syringes with other drug addicts, Renton's test comes back negative.
Clean of heroin, Renton decides to visit Tommy who, by now, is lost in his heroin addiction and has contracted HIV. He's living in a filthy flat. On Diane's advice, Renton moves to London and gets a job as a property letting agent. He begins enjoying his new life of sobriety but it's short-lived after Begbie's involved in an armed robbery. He turns up at Renton's flat, asking for a place to hide out. Sick Boy, who now sees himself as a well-connected pimp and drug pusher, also shows up on Renton's doorstep. Renton's 'friends' make his life miserable, stealing from him and trashing his flat. Seeking to rid himself of them, Renton gives them a flat he's responsible for which they use as a base to commit theft, however, Renton doesn't rent it to them and they live there as squatters. The three soon learn of Tommy's death due to toxoplasmosis and travel back to Edinburgh for his funeral.
Back home, they meet Spud who's been released from prison. Sick Boy suggests a dangerous but profitable heroin transaction. Sick Boy needs Renton's help to supply half of the initial £4,000 supply. After the purchase, Renton checks the purity by injection. The four then sell the drugs to the dealer for £16,000. They all go to a pub to celebrate, all them friendlier and happier than they usually are, and discuss their plans to spend the money. Renton suggests to Spud that they steal the money and run as Begbie & Sick Boy get drinks in. Spud is too scared of Begbie to consider it. Renton believes that neither Begbie nor Sick Boy deserve the money. As Begbie heads back to the table with another round of pints he bumps into another patron and spills much of the beer he bought. When the man apologizes, Begbie turns violent and smashes the man's face with one of the glasses. As he moves in to cut the man with a knife, he slashes Spud's hand. The incident is enough for Renton to decide to later steal the money since he feels he can't relate to either Begbie or Sick Boy any longer.
Early in the morning, Renton wakes up before anyone else and takes the money from Begbie, leaving quietly. Spud sees him leave but says nothing; Renton gives him an affirmative nod. After Begbie discovers the money has gone, he trashes the hotel room in a violent rage that alerts the police, presumably leading to his arrest.
Renton travels to London and vows to live the traditional, stable life he talked about in the beginning of the film. In voiceover, he talks about how Sick Boy would have stolen the money from everyone if he'd thought to do it first. He does feel some remorse for leaving Spud behind and leaves him £2,000 in the locker where he'd been hiding his passport. In the final scene, Spud retrieves the money Renton left for him.