Doctor William Magnus, Colonel Henry Caspar and the Metal Men land upon the roof of a downtown high-rise. It is the last known location of famed chemist, Professor Ramsey Norton. The team has barely had the chance to begin their investigation when the roof top begins to violently shake. A massive hand bursts through the rooftop, blindly grasping Magnus and Platinum. Before they can attempt a rescue, Magnus orders the Metal Men to flee to the skyscraper across the street. Iron and Lead mold themselves into support girders, from which Tin anchors himself. Holding onto Tin with their legs, Gold and Mercury stretch back across the street, to try to pull Magnus and Platinum free from the hand. The strain of Gold and Mercury's pulling causes Tin to buckle. Though Platinum could easily free herself, she chooses to stay with Magnus. The two are drawn back into the building.
Once inside, they discover that the hand belongs to Norton, grown to fantastic proportions. With only seconds of life remaining, Norton speaks his final words. Norton had been conducting countless chemical experiments, all to improve mankind. The chemical combinations from his failed experiments were poured into a giant plastic mold of a man, that Norton referred to as "Chemo". Searching for a solution to world food shortages, Norton discovered a chemical combination that caused rapid growth in organics. Unfortunately, the organisms accelerated growth was too rapid, causing them to explode. Pouring this chemical failure into "Chemo" filled the plastic mold to the top. Norton awoke from a brief nap to find Chemo growing. As Norton approached to investigate, Chemo spat the accelerated growth chemicals on Norton.
His own enormous weight pinning him to the floor, Norton could only watch, helplessly, as Chemo destroyed everything in Norton's laboratory. Then the chemical horror burned it's way out of the building, leaving Norton to die. With his dying breath, Norton begs Magnus to destroy Chemo. Platinum carries Magnus back up, and out of Norton's laboratory, to rejoin the Metal Men. Suddenly, Chemo appears and begins burning down the skeletal skyscraper the team is perched on. Using Lead as her anchor, Platinum spins herself across to the next building, as a wire bridge. The Metal Men get Magnus and Caspar across, just as the skyscraper comes crashing down. To Magnus' horror, Platinum and Lead go down with the skyscraper. Iron molds himself into a crane, to clear the steel debris away from the downed Metal Men. The team returns to Magnus' laboratory complex, where Magnus begins repairs on Lead and Platinum.
The Metal Men are sent out into the field to take down Chemo. Mercury opts to go it alone, believing himself superior to his fellow Metal Men. After melting the tracks in front of a speeding train, Chemo encounters Mercury. The hot-headed Metal Man manages to dodge Chemo's first couple of volleys. The third expulsion, though, reduces Mercury to hundreds of inert globules, that Chemo tramples over. With Mercury's defeat, the other Metal Men move to engage the chemical monstrosity. Tin molds himself into a cup to imprison Chemo. The irresistible chemical creature easily burns it's way through Tin. Iron and Gold flank Chemo, and attempt to puncture Chemo's legs. Exposure to Chemo's chemical make-up causes Iron to rapidly rust, while Gold rapidly tarnishes. Caspar informs Magnus that the Metal Men have failed to stop Chemo. With Lead still out of commission, Magnus only has Platinum to fight by his side.
They locate Chemo, in the midst of destroying another building. Magnus manages to snag Chemo on the Jetaway, and carries the chemical horror out of the city. When the controls of the Jetaway begin to corrode, Magnus is forced to put the ship down. Back on solid ground, Chemo is quick to attack. Magnus is barely able to pull Platinum out of Chemo's line of fire. Chemo's corrosive blast burns so deeply into the Earth that it reaches a natural gas deposit. The super-heated gas bursts upwards, out of the Earth, briefly forming a towering column, before it dissipates. Timing the bursts, Magnus and Platinum leap down into the Earth. They discover a cavern honeycombed with powerful gas jets. Chemo pursues them. Magnus and Platinum just miss being crushed by a pair of gas jets. Chemo, however, isn't so lucky. The giant chemical horror is held fast, as if in a vice, by the two powerful gas jets.
With the pressure of the gas jets holding Chemo prisoner, Magnus and Platinum return to the surface. Magnus gathers the pieces of his Metal Men. Returning to his laboratory, Magnus restores the Metal Men to full operating capacity.