After convict Mike Fallon escapes from prison, he reads about the government testing a new atomic radiation-proof suit of armor. Figuring the police would never think he'd work in such plain sight, Fallon signs on as a test subject under a phony name. He is given the job of working inside the armor, dubbed Metallo, and testing it out.

Realizing how powerful the suit of armor is, Fallon finally reveals his true motives and threatens to take over the world. However, he has started getting terrible pains in his stomach, which a doctor ascribes to being cancer. Fallon realizes that he would have to leave the attack-proof suit to receive treatments, and thus be captured again.

Instead, he opts to walk out into the middle of nowhere, to let himself die in the armor.