Mark Renton

In the late 1980s, Mark Renton is an unemployed heroin addict living in the Leith district of Edinburgh. He attempts to get off heroin, but finds it too difficult and resorts to buying opium suppositories from untrustworthy dealer Mikey Forrester. Later, at a party with his friends Simon "Sick Boy" Williamson, Francis Begbie, Danny "Spud" Murphy, Stevie and Tommy, Dawn, the infant daughter of Simon and a woman named Lesley, dies in her crib, greatly upsetting Sick Boy and inspiring him to get clean from heroin. After witnessing Dawn's death, Renton resumes heroin use.

After continuing to collect unemployment pay, Renton later meets with Tommy, who is now depressed due to being dumped by his girlfriend. He asks Renton to supply him with heroin to make him feel better. Spud and Renton are arrested while stealing. Danny is sent to prison, but Renton avoids jail time due to attending rehab. He hints that the underlying cause of his addiction was from the death of his catatonic brother Davie. Soon after, Renton's other brother Billy is killed by a bombing attack by the Provisional Irish Republican Army. Renton relapses into heroin once more and moves to London, where he has an encounter with a homosexual Italian man named Gi. He later goes back to Edinburgh for the funeral of Matty, a fellow addict.

After witnessing so much death related to heroin, he decides to get clean again while in London, but returns back to Edinburgh again to visit his former dealer Johnny Swan, who recently had his leg amputated from heroin abuse, and Tommy, who is dying of AIDS. He meets once again with Spud, Begbie and Sick Boy, as well as another friend "Second Prize", and the five sell a large amount of heroin to a dealer for £16,000. After celebrating in a pub, Renton decides to steal the money from his friends and walk away to a new life, leaving a small share for Spud.

Several years later, Renton is a now nightclub owner in Amsterdam and has not used heroin in years. One night, Renton sees Carl Ewart, a DJ from Edinburgh (a character from Welsh's novel Glue), who recognises him. Later, Sick Boy learns from some of Carl's friends that Renton now lives in Amsterdam, and the two agree to meet in Zurich, but Renton instead flees to San Francisco. Renton visits his hometown of Leith one more time, but he is seen by Begbie, who is still bitter over Renton stealing their money years earlier. He runs across the road in an attempt to attack Renton, but Begbie is hit by a car.

Renton takes Begbie to hospital and flees Edinburgh for the last time with £60,000 which he stole from Sick Boy, and two of his friends Nikki and Diane. Sick Boy visits Begbie in the hospital as the two plot to find and kill Renton and take back their stolen money.

The film follows much of the plot laid out by the novel, with some changes.

In the film, Mark Renton is a 26-year-old, unemployed man who lives with his parents in Edinburgh, engaging in heroin use with Sick Boy (Jonny Lee Miller), Spud (Ewen Bremner) and their dealer Swanney "Mother Superior" (Peter Mullan). Begbie (Robert Carlyle) and Tommy (Kevin McKidd) urge Renton to cease heroin use and get clean, which Renton unsuccessfully tries to do with the use of opium suppositories. One night at a club, he meets a girl called Diane (Kelly Macdonald), who he later has sex with at her house. He learns the next morning that she is below the age of consent. She threatens to report him to the police if he does not continue the relationship.

Tommy begins using heroin after being dumped by girlfriend, and Renton begins to relapse into heroin use after the death of Dawn. Renton, Spud and Sick Boy are caught shoplifting and Renton is sent to rehab while Spud is sent to prison and Sick Boy narrowly escapes. He becomes depressed due to having no meaning to his life and quickly relapses, but nearly dies from an overdose in Swanney's home. He withdraws from heroin cold turkey and later visits Tommy, who has been infected with AIDS. He later moves to London on Diane's advice and gets a job as real estate agent. Soon after, he is tracked down by Begbie, who is a fugitive back home after stealing from a jewellery store, and Sick Boy, now a pimp and drug dealer.

They return home to attend the funeral of Tommy, who has died of toxoplasmosis. They reunite with the recently released Spud, and the four buy £4,000 worth of heroin from Mikey Forrester, which they resell to a high-level dealer in London for £16,000. They celebrate at a pub, but Renton feels alienated from his friends after Sick Boy indicates that he would steal the money and Begbie needlessly assaults an innocent man after his suit is stained by beer. Renton asks the meek Spud if he would be willing to steal the money for themselves, but Spud fears the consequences.

While getting some rest at a hotel room, Renton decides to take the money and leave. He leaves £4,000 in a safety deposit box for Spud to collect. Begbie violently destroys the hotel room after learning Renton has betrayed them, which gets him arrested by the police. Meanwhile, Renton walks away to his new life.

20 years later, Renton is now 46 and lives in Amsterdam with his wife. On the verge of divorce and a mid-life crisis, he returns to a nostalgic trip to Edinburgh. He visits Spud and saves him from a suicide attempt. Spud tells him where to find Sick Boy and sees him at his pub. Sick Boy angrily attacks Renton, still vengeful for the betrayal. He gives £4,000 to Sick Boy, but Sick Boy angrily rants to his girlfriend Veronika that he is not satisfied and will string Renton along in an elaborate plan to gain his revenge.

Meanwhile, Begbie escapes prison. Renton and Sick Boy begin development of a brothel in the upper floor of Sick Boy's pub, and Renton begins an affair with Veronika. Spud also joins in on the scheme. Although, Sick Boy is initially still mad at Renton for stealing the money, they later bond and resume their friendship. Renton later encounters Begbie at a club, and narrowly survives a stabbing attempt from Begbie. The plot thickens as Begbie visits Spud and Veronika, stealing Veronika's phone and telling Renton and Sick Boy to come to the pub. Spud arrives too late to tell the duo of Begbie's plan, and Sick Boy is knocked out by Begbie after protesting. Renton and Begbie struggle on the top floor of the pub, which leaves Renton hanging by his neck from electrical wiring. Begbie tries to strangle Renton, but he is knocked out by Spud with a toilet bowl.

They leave Begbie outside a prison. Renton and Sick Boy resume their friendship, while Veronika returns back to Bulgaria and Spud reconciles with his family. Renton returns to his father's home and they embrace, with Renton going back to his childhood bedroom and dancing to remix of "Lust for Life".