Doctor William Magnus attempts to repair Platinum's faulty responsometer. Magnus is unsuccessful. Suddenly, Magnus' therapist, Isobel Sullivan, her roommate, Johanna Rome, and Mona Bennet, barge into Magnus' laboratory. Bennet is in trouble, and Sullivan is hoping Magnus can help her. Bennet tells the tale of Doctor Bruce Gordon, her fiance. Gordon had been working on the construction of Solar City, when it was destroyed by Eclipso. Weeks later, Bennet discovered that Gordon was Eclipso. The wound he received from a black diamond had altered Gordon's biochemistry. Under any eclipsed sphere of light, Gordon transformed, against his will, into Eclipso.
After Eclipso suffered a defeat, at the hands of the Justice League of America, Gordon attempted to destroy Eclipso, once and for all. Gordon bombarded himself with high intensity light, Eclipso's one weakness, hoping it would burn Eclipso out of him. Due to a freak accident, Gordon's mechanism shattered. A piece of debris eclipsed the lamp's light, transforming Gordon into Eclipso. Entering Gordon' study to retrieve his weapon, a black diamond, Eclipso discovered an unopened letter, to Gordon, from Bennet's father, Doctor Simon Bennet. Upon reading the letter, Eclipso left, immediately, for the Georgia Augusta University Library in Gottingen, Germany.
Magnus, Sullivan, Rome, Bennet, and the Metal Men, travel to Germany, in the Jetaway. En route, Bennet reveals the contents of her father's letter. In the mid 1500's, a group of Spanish conquistadores stumbled upon the ruins of a lost city, in Peru. Only one of the conquistadores got out of that city alive. He was found, screaming in agony, by a pair of missionary priests. Only a series of strange documents survived the expedition. Upon returning these documents to Spain, they were destroyed, as heretical, by the Spanish Inquisition. One of these documents was saved by a rebellious monk, who fled Spain, to escape the religious persecution.
The document was brought to Germany, ultimately ending up in the Univerisity Library. Storming the library, the Metal Men encounter Eclipso. Unleashing a powerful beam, through his black diamond, Eclipso severs Mercury's arm. Lead blocks the beam from hitting his fellow Metal Men. Tin tackles Eclipso, only to be hurled away, Iron clamps himself around Eclipso, but is beat down by Eclipso's superior physicality. Using his black diamond to plunge Magnus' group into darkness, Eclipso makes his getaway. Drawn to the dying librarian, the team find a map to the Peruvian Andes. En route to Peru, Magnus reattaches Mercury's severed limb.
Following the map, the Metal Men arrive at the same lost city, discovered by the conquistadores. There, they are ambushed by Eclipso. The team scatters, with Tin leaping back into the Jetaway. Tin lures Eclipso in close, then hits him in the face with a welding torch. The bright light of the torch causes Eclipso to transform back into Gordon. With Gordon leading the way, the team depart for Diablo Island, the place where Gordon received the black diamond wound. The island was once home to a sun cult, lead by their chieftain, Mophir. Gordon leads the group into Mophir's cave. Iron finds a stone tablet, inscribed with an eclipsed sun. Picking the tablet up causes an earth tremor.
The group hurries out of the cave, for fear of a collapse. Magnus orders Iron to move the Jetaway away from the cave's mouth. As the Jetaway passes overhead, it momentarily blocks the sun's rays over Gordon. As the shadow fall upon him, Gordon once again becomes Eclipso. Before anyone can react, Eclipso snatches the tablet from Magnus, and scrambles up a mountain peak. The Metal Men give chase. Eclipso begins an incantation. Upon finishing it, he throws the tablet down. Using his black diamond, he cleaves a path into the sea, by boiling the ocean waters. Borne by Mercury's elongating form, Iron slams into Eclipso, knocking him from his perch. It's far too late, though. Eclipso has awoken the ancient God, Umbra, and the world itself is now in jeopardy.