The Legion of Superheroes are a group of teenaged heroes and older in the 31st century.
A Controller released a Sun-Eater to destroy Earth and its sun. The whole Legion and the Legion's worst enemies had to become allies to build the weapon that could destroy the Sun-Eater. In the end, Ferro Lad sacrificed his life to save Earth. Superman, after he warned the Controller to never return, returned to the 21st century, where he became Superman.
Some years after Superman had left (from the 31st century's point of view), Superman-X, a clone of Superman from the 41st century, appeared and required the Legion to follow him. In the 41st century, most parts of the populated worlds had been destroyed by Imperiex, and Superman-X needed the Legion to stop him. The plan backfired however, when Imperiex stole Superman-X's time travel device and went to the 31st century.
When the Legion followed, they were a week too late, and New Metropolis was almost devastated, however, not by Imperiex, but of the Legion of Super-Villains and of the Fatal Five. The Legion of Super-Villains and Fatal Five were, with few exceptions, quickly put back to their prison planet Takron-Galtos. Imperiex allied himself with the Dominators to destroy the Legion of Super-Heroes and the United Planets
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