Superman [Clark Kent]; The Flash [Barry Allen]; Legion of Super-Heroes [Superboy; Brainiac 5; Wildfire; Mon-El; Cosmic Boy; Princess Projectra]; Reverse-Flash [aka Professor Zoom, Eobard Thawne]; Iylar; a Zelkot; a Volkir
In the 25th century, Superman secretly helps Flash deal with Professor Zoom, before the two head off again across time after Iylar, who is trying to reach the past and end an alien civil war before it began. Flash wants to make sure Iylar makes it through time, so that the Zelkot do not destroy Earth. Superman wants to ensure the war begins so that the Volkir visit Earth in the past and seed life on it. After a number of stops across time, Superman and Flash catch up to Iylar, stop him from preventing the war, and neutralize the aliens' threat against the Earth in the present.