During the opening of a museum, Superman tries to stop two robbers. He fails when he falls victim to the poisoning effects of Kryptonite, one of the exhibited rocks. Lex Luthor sees the surveillance tape of the event and determines that this must be Superman's one and only weakness. Luthor orchestrates a scheme to trap Superman, place him in vicinity of the rock, and put an end to him once and for all.

Thanks to the orchestrations of Luthor, terrorist John Corben becomes Metallo, a skin-covered cyborg with a Kryptonite heart. Luthor then sets the supervillain on a quest to kill Superman, but Corben begins to question his role as Metallo when he lacks any human senses in the progress.