The movie returns to the scenes of Trainspotting 20 years after the original story. The main character, Mark Renton, returns from Amsterdam to Edinburgh after some personal setbacks. He meets his old friends, of whom one, Spud, has a severe heroin addiction after his life crashed around him and the other, Sick Boy,
While Mark manages to save Spud from killing himself, the reunion with Sick Boy goes less ideal and ends in a fight. Renton, having stolen 16000 pounds from his friends earlier, gives back Sick Boy's share, starting to heal old wounds. In the meantime, Begbie escapes from prison and forces his well to do son to commit some criminal activity with him. He is driven by revenge on Mark due to his betrayal 20 years earlier.
To help his old friends, Mark, together with Sick Boy and Veronika, apply for a development fund over 100.000 pounds from the European Union with false papers to save the pub by transforming part of it into a brothel. Veronika befriends Spud during the renovations and give him the incentive to write his memoirs. The whole plan goes wrong when their fraud is revealed and the proceeds are eaten up in legal fees and addiction. Begbie happens to clash with Mark in a night club, which the latter only barely escapes alive. An owner of another brothel kidnaps Mark and Sick Boy, forcing them to abandon their plans and hike back to Edinburgh naked.
Meanwhile, Spud is visited by Begbie. He discovers Spud's started memoirs and finds out that Mark has returned the money. Veronika arrives at Spuds. Begbie uses the situation and texts the other two from her phone to come to the pub at midnight, planning an attack. Veronika, afraid for her life asks Spud to run away with her and give his part of the money to his estranged family. They forge signatures to get the money into her account. Begbie continues reading the memoirs and finds out about the horrible alcoholism of his father. He goes back to his exes and apologizes to his son.
Mark and Sick Boy arrive at the pub, where Sick Boy gets immediately knocked unconscious by Begbie. Spud arrives too late to save them. Begbie chases Mark and manages to throw him through the half done floor upstairs, entangling him in some electrical wiring. Before he can finish him off, Sick Boy reappears and incapacitates him with pepper spray. Finally, Spud knocks him out with a toilet. They leave him unconscious in front of the prison inside a car.
The movie ends with Veronika returning to Bulgaria, Spud reconciles with his family and starts to write his memoirs for real. Mark moves in with his old father and resumes his friendships like before.
