A young man, intent on suicide, holds a woman at gun point, while he forces a police officer to cut through the fencing barricade around the top of the Empire State Building. Handing the woman off to the officer, the young man steps through the hole in the fencing barricade, and jumps off the roof. By chance, at that exact moment, the Metal Men are passing by in their flying saucer. With Tin piloting the ship, Lead holds Gold's legs inside the craft, while Gold stretches the rest of his body out, shaping his golden form into a giant net, to catch the plummeting youth. Gold pulls the man aboard their ship. The youth immediately begins firing at the Metal Men. Iron blocks the gunfire, while Platinum winds herself around the man, subduing him. In a hidden chamber, located beneath New Mexico's Carlsbad Caverns, Otto Brisbane overrides the Metal Men's responsometers, drawing them to his location. In Metropolis, the robbery of a high society penthouse bash, via hot air balloon, is thwarted by Superman. The Metal Men awaken, within Carlsbad Caverns, with no memory of how they arrived. Brisbane announces himself, stating that his experiments are about to begin. Suddenly, the walls of Carlsbad Caverns disappear, replaced by the ruins of a large metropolitan city. The Metal Men find themselves under attack from a horde of nightmarish creatures. In short order, the Metal Men subdue the horrific monstrosities.
At the Daily Planet, Clark Kent receives word, from Jimmy Olsen, that the Metal Men have gone missing. Brisbane changes the environment in his experiment to a subzero wasteland. The extreme drop in temperature proves to be especially problematic for Mercury, who tends to be liquid at room temperature, and Tin, who begins to crumple. Iron, Lead, Gold, and Platinum fashion themselves into a mirror to catch, reflect, and focus the rays of the "sun" onto Mercury and Tin. Deeming the pair "unsuitable", Brisbane teleports Mercury and Tin out of his test chamber. Kent's computer, at the Daily Planet, is plagued with glitches. Studying them with his super-vision, Kent sees images of the Metal Men. Though unaware of how his computer could be picking up such signals, Kent, as Superman, decides to follow the impulses back to their source. The Metal Men are next subjected to environmental conditions of extreme heat. Almost immediately, Lead begins to melt. Superman smashes into the chamber, then crashes to the floor. The simulated "sun" is actually a red giant, which robs Superman of much of his powers. Brisbane is crestfallen that Superman should fall so quickly.
Fearing the extinction of humankind, Brisbane's experiments have been designed to ferret out the one form of life capable of surviving the apocalypse, in whatever form it comes. Superman and the Metal Men have all proven to be unsuitable. Suddenly, inspiration strikes Brisbane, who begins the next stage of his experiments. Superman, Gold, Lead, Iron, and Platinum are teleported to Mercury and Tin's location. Superman and the Metal Men set off in a straight line, cutting their way through the jungle, until they reach a metal barrier. Lead and Gold meld themselves into a crane, with Iron as their wrecking ball. The impact of Iron hitting the wall weakens it enough for Superman to break through to the other side. They enter into a chamber where Brisbane is in the process of mutating cockroaches into men. Swarmed by the cockroach creatures, Superman and the Metal Men are quickly overwhelmed. Iron hurls Platinum into the "sun", destroying the red sun radiation projector. With Superman's powers returned to him, the tide of battle quickly turns. Superman and the Metal Men break into Brisbane's command center, just as Brisbane detonates a nuclear warhead, which destroys all the cockroach humanoids. Mercury subdues Brisbane, while Lead destroys Brisbane's computers. Superman points out that, had Brisbane succeeded in mutating all of humanity into his cockroach horrors, Brisbane himself would have been responsible for bringing about mankind's extinction.