Eclipso has just raised an ancient dark God, from the surrounding waters off Diablo Island. In return, the God, Umbra, increases Eclipso's power a thousandfold. Power Eclipso puts to good use, beating back the Metal Men. The Metal Men look to their creator, Doctor William Magus, for leadership, Magnus, though, seems shellshocked from Eclipso's barrage. Eclipso chases after Mona Bennet, Isobel Sullivan, and Johanna Rome. Gold weaves himself about Eclipso's legs, tripping him. In the time it takes Eclipso to fend off Gold, Platinum has reached the women. Eclipso knocks a tree over, which drags Bennet from Platinum's arms. Eclipso grabs Bennet, and carries her up to the cliffside, with the Metal Men in hot pursuit. Eclipso is about to sacrifice Bennet to Umbra, when the Metal Men catch up to him.
Tin and Lead reach Eclipso first, but Tin is easily dodged, while Lead is reduced to molten slag. Gold gathers up Lead's form. With Iron's assistance, they reshape Lead into a convex lead-glass lens. Focusing the sun's light through the lens, the Metal Men blast Eclipso with his one weakness, high intensity light. Eclipso transforms back into his alter-ego, Doctor Bruce Gordon. While Umbra's power builds, Gordon provides the team with a little history lesson. When the Earth was in it's infancy, Umbra, along with other incredibly powerful beings, came to Earth, and were worshipped as Gods. Eventually, tribal mystics were able to drive these "Gods" into exile, or imprison them. Umbra's followers, however, kept possession of a black diamond, which was a fragment of the larger jewel Umbra wore on his forehead.
The black diamond became a kind of "Serving Stone" for Umbra's cult leaders. Their priests used it as a weapon to keep the cult in line, always with an eye toward bringing Umbra back from oblivion. The last of these priests was Mophir. Gordon fought Mophir to the death. Though Gordon was victorious, Mophir managed to scratch Gordon with the black diamond, which led to Gordon becoming Eclipso. Not being a member of the cult, Gordon, and thus Eclipso, didn't know how to summon Umbra. Hence the gathering of certain parchments and stone tablets. Still floating in the ocean, Umbra reaches for Mophir's cave, clawing his way through earth and stone. Believing the Metal Men represent an evolved form of humanity, Umbra sends out five lethal beams targeting the five humans.
Magnus, Gordon, Bennet, Rome, and Sullivan do their best to evade the beams, but ultimately, they find their mark. As the beams cross their own paths, however, Gordon's transformation into Eclipso is triggered. The Metal Men are horrified at the death of their creator, and his companions. Umbra sends out five more beams targeting the Metal Men, intended to enslave them. Umbra's power, however, doesn't work on robots. In defiance, the Metal Men mold themselves into a giant sling, and hurl a boulder at Umbra. Suddenly, Eclipso reappears and attack's Umbra. It seems Eclipso did not appreciate Umbra's attempt to murder him. In truth, Umbra is unaware that Eclipso is also Gordon. Umbra removes the power he bestowed upon Eclipso. The Metal Men rescue Eclipso. Magnus forces their former adversary to add his power to the Metal Men's.
The Metal Men mold themselves into a giant laser cannon, which Eclipso focuses the energy of his black diamond into, powering it up. Targeting the jewel on Umbra's head, the laser drives Umbra back below the sea. The bright flash of the laser transforms Eclipso back into Gordon. With the Metal Men molding themselves into a submarine about Magnus, the famed inventor pursues Umbra beneath the waves. Umbra lays recovering in a deep pit. Magnus co-ordinates the efforts of the Metal Men, to restore several flat stones circling the pit into their proper configuration. Once done, Umbra is imprisoned anew, between dimensions. Returning to the surface, the Metal Men are astonished to see Bennet, Rome, and Sullivan, like Magnus and Gordon, still alive. Platinum reveals that, at the last moment, she wove herself into a protective cocoon around the humans, shielding them from Umbra's power. With the crisis over, the team returns home.