The full story
Spider-Man 3 opens with Peter Parker at his peak. New York loves Spider-Man. He's planning to propose to Mary Jane. His confidence borders on arrogance. Three threats converge to shatter everything.
Harry Osborn, having discovered his father's Green Goblin arsenal, attacks Peter as the New Goblin. Their aerial battle ends when Harry crashes and suffers amnesia, temporarily forgetting his vendetta and rekindling his friendship with Peter. When his memory returns, Harry manipulates Mary Jane into breaking up with Peter, deepening the betrayal.
Escaped convict Flint Marko—a desperate father trying to help his terminally ill daughter—accidentally falls into an experimental particle accelerator and is molecularly fused with sand, becoming the shapeshifting Sandman. The film drops a devastating retcon: Marko was the actual shooter who killed Uncle Ben. Dennis Carradine (the thief Peter failed to stop in the first film) was merely the accomplice. Marko pulled the trigger accidentally during a struggle—he never meant to kill anyone. This revelation destroys Peter, rewriting his origin trauma.
An alien symbiote crash-lands via meteorite and bonds with Peter's Spider-Man suit, turning it black. The symbiote amplifies Peter's powers but also his aggression, arrogance, and cruelty. "Dark Peter" humiliates Eddie Brock—a rival Daily Bugle photographer—by exposing his fabricated photographs, getting him fired. He hurts Mary Jane emotionally. He struts through the city in sequences that are equal parts disturbing and infamous.
Recognizing the symbiote's corruption, Peter tears it off using the vibrations of a church bell tower. The symbiote falls onto Eddie Brock, who is in the church below praying for Peter's destruction. Eddie's hatred fuses perfectly with the alien, creating Venom—a dark mirror of Spider-Man with all his powers, none of his restraint, and a personal vendetta.
Venom allies with Sandman and kidnaps Mary Jane to lure Peter into a final battle. Harry, learning the truth about his father from his butler Bernard, overcomes his hatred and joins Peter. Together they fight Venom and Sandman.
Harry is fatally impaled by his own glider—deflected by Venom—dying in Peter's arms, mirroring his father's death. Peter uses a ring of metal pipes to create sonic vibrations that separate the symbiote from Eddie, then destroys it with one of Harry's pumpkin bombs. Eddie, trying to re-bond with the symbiote, is caught in the explosion and killed.
Sandman, freed from Venom's influence, confesses that Ben's death was an accident and begs for forgiveness. Peter, exhausted and grieving, forgives him. Marko dissolves into sand and drifts away.
The film ends at Harry's funeral, with Peter and Mary Jane reconciling through shared grief. They hold each other, beginning the slow work of repairing what was broken.