The Dominators are planning a sneak attack on Earth while most of the Legion are off-planet! Now it's up to Cosmic Boy and Night Girl to assemble a new Legion of Substitute Heroes in time to block the invasion!

Cosmic Boy and Night Girl arrive on Night Girl's home planet of Kathoon and are asked by a Science Police officer to assist him in a special situation. Cosmic Boy finds that some Dominators are up to no good and calls in for assistance from other members of the Legion of Super-Heroes, but all the active members are engaged in other matters. The person on monitor duty at the time, Bouncing Boy, also had some Legion Academy students along with him. Deciding that he should take the call, Bouncing Boy takes Comet Queen and Myg, the new Karate Kid, with him to Kathoom. They fight the Dominators in a battle, learning that they are trying to sabotage an important meeting between the United Planets and the Dominators, but the Dominators send the assembled heroes into an alternate dimension. They find their way to the Dominator's home world, Dominion, and find a way to stop the criminals from attacking the United Planets. After their mission ends, the assembled heroes decide to band together as the newest incarnation of the Legion of Substitute Heroes, to be there to protect the Earth when the Legion of Super-Heroes are otherwise engaged.