26-year-old Mark Renton is an unemployed heroin addict living with his parents in the suburbs of Edinburgh, Scotland. He regularly partakes in drug use with his friends: Simon “Sick Boy” Williamson, a treacherous, womanising James Bond fanatic; Daniel “Spud” Murphy, an outwardly docile confidence trickster; and Swanney, “Mother Superior”, their dealer. Renton is warned about the dangers of his drug habit by his other friends, Francis “Franco” Begbie, an aggressive psychopath, and Tommy Mackenzie, a footballer, who both abstain from drugs. Growing tired of his reckless lifestyle, Renton attempts to wean himself off heroin with opium suppositories given by dealer Mikey Forrester. At a nightclub, Renton notices that his cessation of heroin use has increased his libido. He woos a girl named Diane Coulston and they have sex in the apartment owned by her parents, but is horrified to learn that she is below the age of consent. Diane uses this as a threat against Renton to continue their relationship.

After several unsuccessful attempts to reintegrate into society, Renton, Sick Boy and Spud relapse into heroin use. Tommy also begins to dabble in drug use, after becoming depressed due to being dumped by his girlfriend. Even the negligence-induced death of Dawn, the infant daughter of Sick Boy and his girlfriend Allison does not convince the group to recover. Later, Renton, Sick Boy and Spud are caught shoplifting; Renton and Spud are arrested while Sick Boy narrowly escapes by running down Princes Street. Spud receives a six-month custodial sentence at HMP Saughton, while Renton avoids jail time due to entering a drug rehabilitation programme. Renton quickly relapses and nearly dies of an overdose at Swanney’s home. Upon returning home after revival at a hospital, Renton’s parents lock him in his childhood bedroom and force him to go cold turkey. Following a difficult withdrawal, Renton is released upon condition that he has an HIV/AIDS test. Despite years of sharing syringes with other addicts, Renton tests negative.

Renton is now clean, but bored and devoid of a sense of meaning in his life. He visits Tommy, who has become severely addicted and has tested HIV-positive. On Diane’s advice, Renton moves to London and takes a job as a property letting agent. He begins to enjoy his new life of sobriety and corresponds with Diane, who keeps him up to date with developments back home. However, to Renton’s exasperation, Begbie tracks him down and takes up refuge with him as Begbie is wanted for armed robbery of a jewellery shop. They are soon joined by Sick Boy, now a pimp and drug dealer. Begbie and Sick Boy later attack two of Renton’s clientele, resulting in him losing his job. The trio then return to Edinburgh for the funeral of Tommy, who has died of AIDS-related toxoplasmosis.

Following the funeral, Sick Boy asks Renton, Begbie and Spud (who has been recently released from prison) help in buying two kilograms of heroin for £4,000 to later resell at a higher price. Renton reluctantly covers the remaining cost, and the group returns to London to sell the heroin to a major dealer for £16,000. As they celebrate in a pub, Renton secretly suggests to Spud that they could both leave with the money, but Spud, motivated by fear of Begbie and loyalty, refuses. However, Sick Boy indicates he would happily do so, and Begbie brutally beats a man after a petty confrontation. Concluding that Begbie and Sick Boy are unpredictable and unable to be trusted, Renton decides to quietly steal the bag of money and leave. Spud witnesses him, but does not warn the others. Renton leaves £4,000 in a safe deposit box for Spud, who “never hurt anybody”. Begbie, seeing Renton and the money are gone, angrily destroys the hotel room where the four stay, prompting the police to arrive and arrest him, while Sick Boy and Spud flee, with the latter claiming his share of the money. Renton walks towards his future, at last “choosing life.”