The plot of the series revolved around a worldwide threat that forces the members of The 99 and the Justice League to work together in order to save the planet. The series took place within the established DC Universe, though it did not feature the members of writer James Robinson's contemporary Justice League, but rather a mix of characters from across the team's long history. Superman, Batman, Wonder Woman (sporting the costume from J. Michael Straczynski's revamp of her title), Hawkman, the John Stewart iteration of Green Lantern, the Barry Allen iteration of The Flash, the Ray Palmer iteration of The Atom, and the Jason Rusch/Ronnie Raymond iteration of Firestorm. Both Vixen and Doctor Light appeared as well, highlighting the multicultural and international aspect of the team.