Doctor Will Magnus, and his astonishing Metal Men, answer questions at the High School of Science. A squabble between Platinum and Mercury embarrasses Magnus. En route home, Magnus questions his value as an inventor. Suddenly, a corrosive beam erupts from the surface of the ocean, cutting an airplane in half. Gold catches the plane, just as it begins to plummet from the sky. Platinum then sews the craft back together, using her own body for the platinum "thread". Taking a good hold of Gold, Iron lifts the airplane up to the Metal Men's jetaway. A second corrosive beam cuts the prow off a Coast Guard vessel. Lead molds himself into a barrier between the aircraft and the sea, taking the full brunt of another corrosive beam. Lead immediately begins to corrode. The Metal Men deliver the aircraft to it's proper destination.
Upon returning to headquarters, Gold, Iron, Tin, and Nameless leap into the metal smelter, to Magnus' horror, destroying themselves out of shame. Platinum, too, leaps into the smelter, but Magnus grabs hold of her. Magnus begs Platinum not to destroy herself. Fearing that Magnus will be pulled into the smelter, Platinum returns to the jetaway. Only Mercury has remained behind, declaring himself the obvious candidate for leadership of the new Metal Men. Magnus recovers what's left of the Metal Men, and begins the long arduous task of reconstructing them. Magnus' examination of the damage to Lead's body leads him to a horrific conclusion. Donning scuba gear, Magnus boards the jetaway, and prepares to return to the site of the attacks. Platinum and Mercury catch up to Magnus before he can depart.
Returning to the attack site, they are almost immediately hit by another corrosive beam, which cuts the jetaway in half. Mercury molds himself into a slide, to save Magnus and Platinum from falling into the ocean. Mercury, though, is splattered into globules by the next corrosive beam. Magnus and Platinum are caught in a powerful vacuum, as they are drawn down beneath the ocean's surface, into the waiting hands of Chemo. Beholding the chemical horror once more, Magnus things back on their first encounter. A scientific colleague of Magnus' had poured thousands of chemical compounds, from failed experiments, into a plastic test tube, shaped like a man. The chemicals, inexplicably, brought the creature to life. It's first act, to attack the scientist with a burst of it's chemical breath.The scientist grew to enormous proportions, living just long enough to tell Magnus his tale.
Magnus and Platinum destroyed Chemo, the first time, with the extreme heat of powerful gas jets. Somehow, chemicals in the seawater have rejuvenated Chemo. As Magnus and Platinum look upon Chemo's glassy face, scenes of Chemo cutting a destructive path across the world play out. Chemo intends to return the world to the seas. A liquid world for a liquid being to rule over. Magnus orders Platinum to return to headquarters, with Mercury, and reactivate the Metal Men. Though it means leaving Magnus behind, Platinum, reluctantly, complies. Mercury gathers the inert bodies of the Metal Men and loads them into the spare jetaway. Before Mercury can reconnect their heads to their proper bodies, Chemo attacks, destroying the jetaway with a single puff of his chemical breath. The Metal Men tumble to the ground. Before they can react, Chemo severs Mercury and Platinum's heads from their bodies, with a single chemical burst.
The decapitated bodies of the Metal men scramble to put their heads back on. Only Tin and Nameless, however, manage to get their own heads on their own bodies. The confusion leads the Metal Men into disaster. Gold finds he cannot spin Lead's non-ductile form into wire, to bind Chemo. Instead, he is consumed by another chemical burst. Iron finds his ability to batter Chemo completely ineffectual using Mercury's liquid form. Lead finds that Gold's body is too soft to form a proper barrier against Chemo, and falls once more to the monster's corrosive beam. Fearing the worst, Platinum runs to Magnus, so that she may die with her creator. In desperation, Tin and Nameless launch themselves at Chemo. The heat of Chemo's form reduce the two robots into tin powder. Completely coating Chemo's form, the creature slowly suffocates to death. Magnus, Mercury, and Platinum collects the remains of the fallen Metal Men. Back in his laboratory, Magnus is able to successfully reconstruct the Metal men.