Flamboyant mob chief Eddie Candell (Ron Randell), who was framed for murder by jealous associate Andy Damon (Anthony Caruso), makes a break while on his way to the death house. Fleeing across the desert, he suddenly finds himself on an atomic testing range, just as an experimental bomb goes off. Instead of being killed instantly, Eddie is bombarded by radiation from Cobalt Isotope X, a newly discovered element that leaves him alive but transformed -- reacting to the radiation and the steel of the handcuffs on his wrists, his body takes on the hardness of steel and can absorb the metal on contact, including any bullets that might be fired at him. In addition to making him all but indestructible, the mutation gives Eddie the strength of ten men, which he uses to tear his way through the ranks of his former associates, terrorizing the woman (Debra Paget) who betrayed him and crushing the life out of anyone who gets in his way as he tries to get to Damon. However, his psyche has been affected as well; he was already consumed by a desire for revenge, but he slowly loses any ability to perceive pleasure or compassion as he slowly transforms into a kind of living metal and the body count around him rises. Only Carla Angelo (Elaine Stewart), his girlfriend and also a genuinely "nice" girl, can reach him, and she must decide whether to help to try and save him or to destroy him.