Clark Kent is terrified by a sight that shakes him to his core. Noticing his obvious distress, Jimmy Olsen and Lois Lane question Kent on the matter. Kent offers a weak rationale, then bolts from the room. Out of sight of his co-workers, Kent changes into his Superman costume, and flies off into space. To his horror, the Sun-Eater is on approach to Earth. The sight of the creature resurrects memories of Superman's first encounter with the creature, when he was a member of the Legion Of Super-Heroes. That time, Ferro Lad heroically sacrificed his life to destroy the Sun-Eater. As Superman closes with the creature, the Sun-Eater bombards him with enough solar energy to shatter a planet. Superman is hurled back through space, eventually coming to rest on the Moon. There, Superman encounters his old foe, Mongul. It is Mongul who has gained possession of the Sun-Eater, slaying the creature's master, a Controller, and setting the beast on a course for Earth's sun. All out of revenge, for Superman having thwarted Mongul's previous schemes. Imprisoning Superman in one of his cube cages, Mongol leaves the Man of Steel on the Moon, to bear witness to the destruction of the Earth. Lois Lane and Jimmy Olsen observe Superman's dilemma through a high powered telescope, as well as the approach of the Sun-Eater.
Lane rushes off to find help. Olsen activates the emergency signal on his Legion Flight Ring. One thousand years in the future, six members of the Legion Of Super-Heroes debate whether or not to answer Olsen's call. Though they risk altering history, ultimately, the team decides to makes the journey back to the 20th century. As the Sun-Eater moves ever closer to Earth's sun, Supergirl, and three members of the Justice League of America, fight a losing battle against Mongul. Directing his power through the Sun-Eater, Mongul triumphs over Black Canary, Green Lantern, Red Tornado, and Supergirl. The Legion Of Super-Heroes arrives on the moon. Their first order of business is to free Superman. With Brainiac 5 and Shadow Lass providing sufficient shielding for the rest of the team, Sun Boy generates the power of a burning star, which shatters the cube cage. Superman and the Legion Of Super-Heroes confront Mongul. In the course of the fight, the Legion's time bubble is destroyed. Only Wildfire's anti-energy barrage really seems to hurt Mongol. Even Element Lad's inertron cocoon fails to impede the would-be conqueror. Wildfire's attack has, unfortunately, also destroyed Mongul's control circuits, leaving him unable to command the Sun-Eater.
The Legion of Super-Heroes break off the attack on Mongul, to go after the Sun-Eater. Superman stays behind to deal with his old foe. Brainiac 5 has Cosmic Boy, Element Lad, and Shadow Lass combine their powers to add strength to his force shield. Then Sun Boy uses his power to draw the Sun-Eater to them. The Legionnaires plunge into the heart of the Sun-Eater. As they approach the core, Wildfire sends the team back the way they came, and proceeds to the core alone. Once Wildfire reaches the heart of the Sun-Eater, he channels the totality of his anti-energy body into one devastating blast. The shockwave from the Sun-Eater's destruction is felt as faraway as Earth. Despite Mongul's greater power, Superman finds the fortitude and will to finally beat the conqueror, once and for all. The Legion return to the moon, where Element Lad and Sun Boy imprison Mongul. Superman scans the heavens for any trace of the Sun-Eater, and can find none. Unfortunately, Wildfire, too is gone. Superman mourns the loss of his old friend, until Brainiac 5 reminds Superman that Wildfire's anti-energy body can never truly be destroyed. Once a new containment suit can be found, Wildfire will rejoin his teammates. Superman gives the exclusive news story to Olsen, no doubt due to his quick thinking in contacting the Legion of Super-Heroes for aid.