The series storyline was a sequel to DC's 1985 limited series "Crisis on Infinite Earths", which "rebooted" much of the DC continuity in an effort to fix 50 years of contradictory character history. It revisited characters and concepts from that earlier "Crisis", including the existence of DC's Multiverse. Some of the characters featured were alternate versions of comic icons such as an alternate Superman named Kal-L, who came from a parallel universe called Earth-Two. A major theme was the nature of heroism, contrasting the often dark and conflicted modern-day heroes with memories of "lighter" and ostensibly more noble and collegial heroes of American comic books' earlier days.