At a moment of crisis, Cosmic Boy returns to lead the Legion, accompanied by a mysterious new hero.

The ancient organic weapon known as the Sun-Eater has been unleashed, and it lives up to its name. It heads toward Earth's sun and not even the Legion can stop it.

The Legion must turn to help from their greatest foes when they are unable to stop the Sun-Eater themselves. Meanwhile, Superman confronts the Sun-Eater's controller but even he may be powerless before the alien.