Henry "Hank" Howard, a government research scientist, has been commissioned by General Darwin to develop a serum that promises enhanced strength and longevity. He hopes to marry Hannah Darwin, but Darwin refuses to bless the union until the serum proves its worth. On a date that starts with a carnival and a late subway ride, a mugger robs Howard of his wallet and the engagement ring he plans to propose with. Howard tries to defend himself but is overpowered, and Hannah notes his sudden courage. To cover for the failed proposal, Howard fabricates that the mugger took a flash drive containing information about the serum.
Desperate to prove the serum’s effectiveness, he injects himself, transforming into a giant purple humanoid—the Bulk. He kills the mugger and bolts as police arrive, leaving behind purple blood as a clue. Det. Ray Garton and Det. Lisa Tuttle discover Howard’s wallet the next day and spot the Bulk at the crime scene, recognizing the connection between the wallet and the strange metamorphosis. They witness the Bulk’s terrifying return to the scene and realize Howard’s role in the crime.
The Bulk unleashes a city-wide rampage, lifting cars and hurling them through the air. One vehicle crashes into Tuttle, crushing her, while Garton fires at the monstrous form. The Bulk also grabs and destroys a news helicopter before morphing back into Howard, who is arrested by Garton. Darwin then visits Howard, sedates him, and takes him to a secret facility where scientists analyze the serum’s effects. Darwin explains that, if Howard agrees to cooperate, he will grant permission for the marriage and provide an antidote—but only after Howard takes down a rogue scientist, Dr. Werner von Kantlove, who has been desecrating monuments and plotting to blow up the moon.
Howard transforms into the Bulk once more and goes on a mission to kill Werner and his dim-witted wife. After the deed, the Bulk returns to Howard, who calls Darwin to report success. Darwin betrays him, admitting there was never a cure and that Werner funded the project to cure his erectile dysfunction; with Howard proven potent, Darwin no longer needs him and plans to use the serum to create a super-human soldier army. He orders a military response and a nuclear bomb is dropped on the Bulk, while Hannah arrives and Darwin tells her that Howard died saving the world.
Hannah awakens to find Howard alive at her house. He proposes again, and she accepts, though she remains wary of Darwin’s true motives. A confrontation ensues when Darwin and Howard clash on a balcony, ending with Howard’s apparent death. Hannah later visits his grave and leaves a rose. A drunken Det. Garton staggers to the grave and, in a crass moment, urinates on it, only for the Bulk’s fists to strike his head.
