Mark Renton, a 26-year-old unemployed heroin addict, lives with his parents in the Edinburgh ward of Leith and regularly takes drugs with his "friends": treacherous, womanising James Bond fanatic Simon "Sick Boy" Williamson; docile and bumbling Daniel "Spud" Murphy; and Swanney—"Mother Superior"—their dealer. Renton's other friends, aggressive, alcoholic psychopath Francis "Franco" Begbie and honest footballer and recreational speed user Tommy Mackenzie, who both abstain from heroin, warn him about his dangerous drug habit. Growing tired of his reckless lifestyle, Renton attempts to wean himself off heroin with a bare room, foodstuffs, and opium suppositories from ill-reputed dealer Mikey Forrester. He develops diarrhoea and has to relieve himself in the disgusting toilet of a betting shop, then imagines himself swimming in the filthy water as he retrieves the suppositories.
Renton attempts to lead a "useful and fulfilling" life away from heroin. This consists of him meeting Sick Boy in a park where he shoots a stranger's dog with an air gun, supplying Spud with amphetamine for a job interview that turns disastrous, and stealing a sex tape of Tommy and his girlfriend, Lizzy. At The Volcano nightclub, Renton notices that his cessation of heroin use has increased his libido. He seduces a girl named Diane Coulston, and they return to her apartment to have sex. The following morning, Renton is horrified to learn that she is below the age of consent and lives with her parents, whom Renton initially assumes are her flatmates. Diane threatens Renton with blackmail to report him to the police for statutory rape if he does not continue the 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 Lizzy, thanks to the actions of an unknowing Renton. Despite the group’s shock, grief, and horror regarding the negligence-induced death of Dawn, the infant daughter of Sick Boy and his girlfriend Allison, the group still does not recover from their heroin use. Later, Renton, Sick Boy, and Spud are caught shoplifting; Renton and Spud are arrested while Sick Boy narrowly escapes. Spud receives a six-month custodial sentence at HMP Saughton, and Renton narrowly avoids jail by entering a drug rehabilitation programme where he is given methadone to help him. However, Renton quickly relapses and nearly dies of a heroin overdose at Swanney's home. Upon returning home after his revival at the hospital, Renton's parents lock him in his childhood bedroom and force him to go cold turkey. Following a horrific withdrawal punctuated by hallucinations of his friends and visions of Dawn crawling on the ceiling, Renton is released upon the condition of an HIV/AIDS test. Despite years of sharing syringes with other addicts, Renton tests negative.
Now clean but bored and devoid of meaning in his life, Renton visits Tommy, who is now severely addicted to heroin and is HIV-positive. On Diane's advice, Renton moves to London and works as a property letting agent. He begins to enjoy his new life of sobriety in London and corresponds with Diane, who updates him on developments back home. To Renton's shock and frustration, Begbie, wanted for a failed armed robbery, tracks him down and takes refuge in his apartment. Sick Boy, now trying to be a pimp and drug dealer, soon joins them. Begbie and Sick Boy later attack two of Renton's clientele (at a supposedly impossible-to-sell property where Renton has sent them to get them off his back), resulting in him losing his job. The trio returns to Edinburgh to avoid police attention and 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) for help in buying two kilograms of pure heroin from Mikey Forrester (who got it after a drunken night out with two Russian sailors), for the low price of £4,000, to sell on, with Renton needing to supply the remaining £2,000 asking price. After Begbie threatens him, Renton reluctantly covers the remaining cost, and the group returns to London to sell the heroin to a dealer for £16,000. As they celebrate in a pub, Renton secretly suggests to Spud that they could leave with the money, but Spud, motivated by fear and loyalty, refuses. Sick Boy indicates he would happily do so, and Begbie brutally beats a man after a minor accident. Concluding that Begbie and Sick Boy are unpredictable and dangerous, Renton quietly steals the bag of money and leaves the following morning. Spud witnesses him but does not warn the others. Renton leaves £4,000 in a left-luggage locker for Spud, who "never hurt anybody". Begbie, discovering Renton and the money gone, enragedly destroys the hotel room where the four stayed, prompting the police to arrive and arrest him as Sick Boy and Spud flee. Spud discreetly claims his share of the money, and Renton walks away to his new life.
