Twenty years after the events of the first film, Mark Renton returns to Edinburgh from his new life in Amsterdam. His return is not triumphant; he's come back due to his mother's death and the realization that life has not unfolded as he hoped. He reunites with old friends Spud and Sick Boy, who are each struggling with their own life issues. Spud has never managed to shake off his heroin addiction despite having a family, leading to despair and a sense of lost time. Sick Boy runs a failing pub while engaging in blackmail schemes, with ambitions to open a brothel.

The reunions are initially awkward and infused with a sense of nostalgia and lost youth. Mark reconnects with his former girlfriend, Diane, now a successful solicitor, and attempts to make amends for his past. However, the shadow of his old life and the betrayal of his friends looms over these rekindled interactions. Meanwhile, Begbie, the most volatile member of their group, has escaped prison and is seeking revenge, completely unaware of Mark's return. The intersecting lives of the four men begin to spiral as old wounds are opened, and old habits threaten to reemerge amidst schemes and a search for redemption.