The show will reair on the Hub in March 20, 2012.
Taking up soon after Justice League ended. It features a greatly expanded League, in which the characters from the original series—now referred to as "founding members"—are joined by a large number of other superheroes from the DC Universe; in the first episode, well over 50 characters appear. A number of these were heroes who had made guest appearances in Justice League, Batman: The Animated Series, and Superman: The Animated Series, but many heroes and other characters made their first animated appearances in this series. The general format of each episode is to have a small ad hoc team assemble to deal with a particular situation, with a focus on both action and character interaction.
Most episodes tell a self-contained story, but the series also features extended story arcs, the first involving the building conflict between the League and a secret government agency known as Project Cadmus. This plot line builds upon events that occurred during the second season of Justice League (which in turn built upon events in Batman: The Animated Series, Superman: The Animated Series, Batman Beyond, Static Shock, and The Zeta Project), and has affected the plotlines of most of its episodes. It was resolved in a four-part story at the end of the first season of Justice League Unlimited. The second and final season story arc focuses on the new Secret Society (which is based on the Legion of Doom) as the main villains, a loose-knit organization formed to combat the increased superhero coordination of the first season.
Towards the end of the series, certain characters became off-limits to the show. Characters associated with Batman and those who appeared in Batman: The Animated Series (aside from Batman himself) were restricted due to the unrelated animated series The Batman to avoid continuity confusion. However, in the episode "Grudge Match", a silhouette of Nightwing can be seen in one shot crouching amongst the stone gargoyles. Aquaman and related characters were unavailable due to the development of a pilot for a live-action series featuring the character as a young man (similar to Smallville), which was not picked up. Characters from DC's "mature readers" Vertigo imprint were also not allowed. No characters from the Teen Titans animated series appeared in JLU, nor vice versa, though Speedy appeared in an episode referencing the Seven Soldiers of Victory after Teen Titans was canceled.
To compensate for this, the producers used previously overlooked DC Comics characters to focus on in the stories. These include characters like Deadman, Warlord, and an unnamed modern equivalent of The Seven Soldiers of Victory. Also, reflecting the previous series, Superman: The Animated Series' focus on Jack Kirby's modern DC characters, Justice League Unlimited similarly focused on the DC/Charlton Comics characters created by Steve Ditko.
DC Comics created an ongoing monthly comic book series based on the TV series, as part of its Johnny DC line of "all ages" comics.
Justice League Unlimited, like the second season of Justice League, is animated in widescreen. The show also features new theme music. The two-part series finale was aired in the UK on February 8 and February 18, 2006 and in the United States on May 6 and May 13, 2006.