On a school trip, high school senior Peter Parker visits a genetics laboratory with his friend Harry Osborn and his crush Mary Jane Watson. There Peter is bitten by a genetically engineered “super spider” and seemingly falls ill after returning home. Meanwhile, Harry’s scientist father Norman Osborn, owner of Oscorp, tries to secure an important military contract. He experiments on himself with an unstable performance-enhancing chemical and goes insane, killing his assistant.
The next morning, Peter finds he is no longer near-sighted and his body has metamorphosized into a more muscular physique. At school, he finds his body can produce webs, and his quickened reflexes let him avoid injury during a confrontation with bully Flash Thompson. Peter discovers he has developed superhuman speed and strength, the ability to stick to surfaces, and a heightened ability to sense danger.
Brushing off his uncle Ben‘s advice that “with great power comes great responsibility“, Peter considers impressing Mary Jane with a car. He enters an underground fighting tournament to raise the money and wins his first match, but the promoter cheats him of his earnings. When a thief suddenly robs the promoter’s office, Peter allows him to escape. Moments later, he discovers Ben was carjacked and killed. Peter pursues and confronts the carjacker, only to realize it was the thief he let escape. After Peter disarms him, the carjacker flees but dies after falling out a window. Meanwhile, a crazed Norman interrupts a military experiment by Oscorp’s corporate rival Quest Aerospace and kills several people.
Upon graduating, Peter begins using his abilities to fight injustice, donning a costume and the persona of Spider-Man. J. Jonah Jameson, publisher of the Daily Bugle newspaper, hires Peter as a freelance photographer, since Peter is the only person providing clear images of Spider-Man. Norman, upon learning Oscorp’s board plans to force him out in order to sell the company to Quest, assassinates them at the World Unity Fair. Jameson dubs the mysterious killer the Green Goblin. Norman (as the Green Goblin) offers Spider-Man a place at his side, but Peter refuses. They fight and Peter is wounded. At Thanksgiving dinner, Peter’s aunt May invites Mary Jane, Harry, and Norman. During the dinner, Norman sees the wound and realizes Peter’s identity. Shortly after he leaves, Norman attacks and hospitalizes May.
Mary Jane admits she is infatuated with Spider-Man, who has rescued her on numerous occasions, and asks Peter whether Spider-Man ever asked about her. Harry, who is dating Mary Jane, arrives and presumes she has feelings for Peter after seeing them hold hands. Devastated, Harry tells his father that Peter loves Mary Jane, unintentionally revealing Spider-Man’s biggest weakness. Norman holds Mary Jane and a Roosevelt Island Tram car full of children hostage alongside the Queensboro Bridge. He forces Peter to choose whom he wants to save and drops Mary Jane and the children. Peter saves both Mary Jane and the tram car, while Norman is jeered by civilians who side with Spider-Man. Norman then grabs Peter, throws him into an abandoned building, and brutally beats him. When Norman boasts about how he will later kill Mary Jane, an enraged Peter overpowers Norman.
Norman reveals himself to Peter, who stops attacking. Norman begs for forgiveness but at the same time controls his glider to try to impale his foe. Warned by his spider-sense, Peter dodges the attack, and the glider fatally impales Norman instead. Norman tells Peter not to reveal his Goblin’s identity to Harry before dying. Peter takes Norman’s body back to the Osborn house, Harry arrives to find him standing over his father’s body. Harry seizes a gun, intent on shooting Peter, but Peter escapes and hides Norman’s equipment.
At Norman’s funeral, Harry swears vengeance toward Spider-Man, whom he deems responsible for his father’s death, and asserts that Peter is all the family he has left. Mary Jane confesses to Peter she is in love with him. Peter, however, feels he must protect her from the unwanted attention of his enemies, so he hides his true feelings and tells Mary Jane that they can only be friends. As Peter leaves the funeral, he recalls Ben’s words and accepts his new responsibility as Spider-Man.