From the introduction:
Supervillains, criminals, aliens, monsters, robots, magicians, demons, gods, there are countless supernatural threats out there that can't be defeated by any single hero. So what happens when earth faces a threat greater than any single hero can handle? Simple, more than one hero comes to the planet's aid. Seven heroes in fact, and a few more later on.
Welcome to the DC Universe, or rather one of the countless universes all bound together in the massive DC multiverse. This particular universe details the animated adventures of Superman, Batman, Wonder Woman, and several other major heroes as they try to defend the world while part of the newly formed Justice League! A team of superheroes dedicated to saving lives and protecting the world against supervillains and extra terrestrial threats.
You'll arrive in this world a few hours before a powerful and merciless alien foe invades the earth and triggers a series of events that'll lead to the formation of the Justice League.