In the newly discovered earth 53 there exist no heroes. That's where doomsday comes in. Doomsday from earth 0 (the main dc universe) decides to travel there and take over the universe for his own gain. That leaves it up to earth 0's justice league to deliver the powers to 7 young heroes who are the only chance to save earth 53. Please do not bash for continuity errors, I do not know Dc as well as marvel.