In Spider-Man 3, Peter Parker is taken over by an alien symbiote, fights Sandman and Harry Osborn, and finally destroys Venom. Spoilers follow for the whole film, which is a chain of three conflicts that all land on Peter at once: a symbiote that changes his personality, a villain tied to his past, and a friend turned enemy.
Peter is enjoying public praise and wants to marry Mary Jane, but her stage career is struggling and the two start to drift apart. The black alien symbiote from the meteorite bonds with Peter's suit, making him stronger but also arrogant and aggressive. Harry attacks Peter as the New Goblin, suffers a head injury and forgets Peter's secret. Police captain Stacy reveals that Flint Marko, now the Sandman, was the real killer of Peter's Uncle Ben.
Driven by rage, Peter fights Sandman in the subway and appears to destroy him. He also humiliates the rival photographer Eddie Brock at the Daily Bugle by exposing his faked photos. Wearing the dark suit, Peter flirts with Gwen Stacy to hurt Mary Jane, treats his friends cruelly and badly injures Harry in another fight. Harry regains his memory and tricks Mary Jane into leaving Peter.
After Peter accidentally strikes Mary Jane, he realizes what the suit is doing to him. He uses the sound of a church bell to tear the symbiote off, but Brock takes it on and becomes Venom. Venom and Sandman team up and seize Mary Jane to lure Spider-Man into a trap. Harry learns the truth about his father's death and joins Peter in the fight.
Harry is fatally wounded while saving Peter, and Peter destroys Venom with a sonic blast and a pumpkin bomb, which also kills Brock. Marko explains that Ben's death was an accident, Peter forgives him, and Marko drifts away as sand. After Harry's funeral, Peter and Mary Jane reconcile and dance together.