Five time lost members of the Legion Of Super-Heroes step through a star gate into the unknown. They are stunned to discover an exact replica of 20th Century Metropolis floating on a chunk of rock in space. Against their will, they are drawn into the city via a powerful tractor beam. Ultra Boy pulls free of the beam, and flies down to destroy it's emitter. He is immediately attacked by Superman. After defeating Ultra Boy, Superman turns towards the descending Legionnaires, bathing them in his heat vision. Element Lad creates a lead shied around the team to protect them from direct exposure, though the temperature inside the sphere continues to rise. Chameleon Boy, Phantom Girl, and Shrinking Violet exit the sphere, each in their own fashion, to confront Superman. To their surprise, a trio of Supermen are waiting for them. With the Legionnaires engaged in combat with the multiple Supermans, the architect of this scenario, Brainiac, deems it time to broadcast a signal to the real Superman. After signing off the air, anchorman Clark Kent receives Brainiac's ultrasonic broadcast, and investigates it's origin point, as Superman.
Chameleon Boy nets the three Supermen, to give Phantom Girl and Shrinking Violet the chance to escape. A revived Ultra Boy returns to the fight, and reveals that the Supermen are actually robots. After Ultra Boy and Chameleon Boy have destroyed the Superman robots, Chameleon Boy frees Element Lad from the lead sphere. The Legionnaires regroup, only to be surrounded by an army of Superman robots. Attacked en masse, the Legionnaires fight back, finding the Superman robots fairly easy to destroy. Having passed through several dimensions, in search of the signal's origin point, the real Superman arrives at Brainiac's Metropolis. A quick scan of his X-Ray vision reveals that every detail of the Metropolis replica is completely authentic, with the exception of the anachronistic presence of the Legionnaires. As Superman approaches their position, in the Galaxy Broadcasting Building, he is joined by a large group of the Superman robots. Superman realizes, too late, the the Legionnaires will interpret their arrival as an attack.
Superman attempts to reason with the Legionnaires but they take him for just another robot, albeit a more sophisticated one with speech capabilities. Ultra Boy and Chameleon Boy beat Superman into unconsciousness. With Superman's defeat, Brainiac reveals itself. The Legionnaires learn that, upon their arrival in the Metropolis replica, Brainiac's computers had subtly seized control of their minds, guaranteeing they would react violently upon seeing Superman. In fact, they are still in Brainiac's thrall, and thus do nothing as Brainiac loads Superman into a molecular disintegrator. Having no further use for the Legionnaires, Brainiac focuses on killing Superman. With Brainiac's control gone, the Legionnaires set about trying to free the Man of Steel. Brainiac's force field, however, keeps Ultra Boy at bay. Element Lad points out to Brainiac that the molecular disintegrator seems to be backwashing over Brainiac. Seeing it's own metal limbs beginning to smoke, Brainiac destroys the molecular disintegrator, teleporting away under cover of the escaping gases. Element Lad reveals that he had used his ability to transmute elements on Brainiac, to trick it into believing it was being vaporized. The crisis over, the Legionnaires make ready to return to their century.