The film opens with Mark Renton (Ewan McGregor) and Spud (Ewen Bremner) sprinting down Princes Street, chased by security guards. Renton reflects on his choices, asserting that while most people "choose life" with its conventional pursuits—children, financial security, and material possessions—he has opted for the chaotic existence of a heroin addict. The narrative introduces his circle of friends, which includes the unscrupulous Sick Boy (Jonny Lee Miller), the ambitious athlete Tommy (Kevin McKidd), the naïve but well-meaning Spud, and the aggressive sociopath Begbie (Robert Carlyle). All of them share a penchant for heroin, often congregating at the apartment of their drug dealer, "Mother Superior" Swanney (Peter Mullan).
After waking from a heroin stupor, Renton resolves to quit using. He meticulously constructs a plan, locking himself away in a decrepit flat, boarding up the door to prevent any potential escape. Realizing he craves one last high, he ventures out to procure opium rectal suppositories from Mikey Forrester (Irvine Welsh). Attempting to reconnect with his friends in a sober state, Renton finds himself feeling incomplete. His night out at a club rekindles his libido, leading him to a young woman named Diane (Kelly MacDonald)—only to discover, after a passionate encounter, that she is a fifteen-year-old schoolgirl living with her parents. Ashamed of the situation, Renton tries to distance himself, but Diane’s blackmail forces him to maintain contact.
Meanwhile, Tommy suffers emotionally after being jilted by his girlfriend Lizzy. Renton had a hand in this, having stolen and hidden one of their intimate tapes, inadvertently damaging their relationship. The cycle of addiction resumes as Renton, Sick Boy, and Spud fall back into heroin use, pulling Tommy down with them. Their destructive lifestyle takes a tragic turn when their friend Allison, a fellow addict, discovers that her infant daughter, Dawn, has died due to neglect. The loss sends shockwaves through the group, particularly affecting Sick Boy, who is implied to be the child's father.
In the wake of this tragedy, Renton recounts how the child's death fuels Sick Boy's self-destructive behavior. As their situation spirals further out of control, Renton narrates the events that lead to Spud's arrest and his own entry into a drug intervention program, where he receives Methadone. Despite his family's support, Renton's depression deepens, prompting an escape back to Swanney's flat, where he nearly overdoses. After surviving this near-fatal experience, Renton's parents confine him to his childhood bedroom during his withdrawal phase.
During this harrowing time, he endures vivid hallucinations involving Diane, visions of his friends, and even the ghostly image of the deceased baby. His ultimate wake-up call arrives when his parents encourage him to get tested for HIV, a looming threat given his past drug use. Remarkably, he tests negative, but this moment of relief is short-lived as he continues to grapple with boredom and despair.
Renton's journey leads him to visit Tommy, who has succumbed to full-fledged addiction and is now HIV positive. Prompted by Diane, he relocates to London, securing a job as a property letting agent and finding a measure of joy in sobriety, even exchanging letters with Diane. But his newfound happiness is upended when Begbie, fresh from an armed robbery, shows up at Renton's flat, seeking refuge. Soon after, Sick Boy arrives as well, complicating Renton's attempts to lead a normal life. The chaotic presence of his "friends" leads to constant strife, with Sick Boy even selling Renton's belongings and pressuring him into dubious schemes.
After learning about Tommy's tragic demise from toxoplasmosis, Renton returns to Edinburgh for the funeral. There, the old crew reunites with Spud, who has been granted parole. Sick Boy proposes a risky heroin deal with Mikey Forrester, and Renton, much against his better judgment, decides to partake. He injects heroin to check the substance's purity but makes a silent vow to resist returning to old habits.
The four friends successfully sell the drugs, but tension builds as Renton becomes increasingly disillusioned with Sick Boy and Begbie. In a pivotal moment that showcases his resolve, Renton decides to betray his friends. Under the cover of night, he slips away with the money, leaving only a £2,000 note for Spud. The film culminates in Renton’s escape; he walks through London, declaring his intention to embrace a traditional and stable life, while Spud discovers the money left behind—a gift for a fresh start.
