Peter Parker plans to propose to Mary Jane Watson, who has just made her Broadway musical debut. While the two stargaze in Central Park, a meteorite lands nearby and an extraterrestrial symbiote follows Peter to his apartment by attaching to his motorbike. Harry Osborn, knowing Peter is Spider-Man, seeks to avenge his father Norman’s death. Using his father’s performance-enhancing gas and Green Goblin technology, he battles Peter, eventually getting knocked out and developing partial amnesia.
Meanwhile, police pursue escaped convict Flint Marko, who visits his wife and sick daughter before fleeing again. Falling into an experimental particle accelerator that fuses his body with the surrounding sand, he gains the ability to control sand and reform his body with it, becoming Sandman. Marko robs an armored truck, and escapes after easily defeating Spider-Man. NYPD Captain George Stacy, Gwen’s father, informs Peter and his aunt May that Marko is uncle Ben’s true killer; the initial suspect, the deceased Dennis Carradine, was Marko’s accomplice. At his apartment, as Peter sleeps in his Spider-Man suit while waiting for Marko to come out of hiding, the symbiote assimilates the suit. Peter later awakens on top of a building, discovering that the symbiote has colored his suit black and enhanced his powers; however, it also brings out his dark side.
Spider-Man locates and battles Marko in a subway tunnel. Discovering that water is Marko’s weakness, he opens a pipe, releasing water that reduces Marko to mud and washes him away. Peter’s changed personality alienates Mary Jane, who is struggling in her career because of her show receiving negative reviews from critics. She shares a tender moment with Harry, but leaves in regret. Urged by a hallucination of his father, Harry recovers from his amnesia and forces Mary Jane to break up with Peter. After Mary Jane tells Peter she loves “somebody else,” Harry meets with Peter and claims to be that person. Under the influence of the symbiote, Peter later confronts Harry over this and spitefully tells him that his father never loved him. As Peter leaves, Harry throws a pumpkin bomb at him, who deflects it back, disfiguring Harry’s face.