Watch Justice League United Online

The series is a continuation of its predecessor, taking up soon after Justice League ended. It is sometimes considered to be the same series as the original; the first season of Justice League Unlimited is referred to by the producers as the third season of Justice League. However, seasons referred to below treat Justice League Unlimited as a separate series, just as it was marketed on DVD.

It features a greatly-expanded League, in which the characters from the original series now referred to as "founding members" are joined by a number of superheroes (in the first episode, well over 50 characters appear), as well as a number of DC heroes who had made guest appearances in the first two seasons of Justice League, Batman: The Animated Series, and Superman: The Animated Series.

Unlike Justice League, which primarily consisted of two- or three-part stories with only light continuity between them, Justice League Unlimited features ongoing story arcs, the first involving the building conflict between the League and a secret government agency known as Project Cadmus. This plot line heavily builds upon events that occurred during the second season of Justice League (which in turn built upon events in The New Batman / Superman Adventures, in which a Darkside-brainwashed Superman almost conquers the Earth single-handedly and awakens the deepest fears of the US Government), and has affected the plotlines of most of its episodes. It was resolved in a four-part story at the end of the second season of Justice League Unlimited. The third season story arc focuses on the Legion of Doom as the main villains, a loose-knit organization formed to combat the increased superhero coordination of the first season.

Another aspect of the Cadmus storyline was its tendency to incorporate elements from the past DCAU Shows (Batman: The Animated Series, Superman: The Animated Series, Batman Beyond, and The Zeta Project).[1] Additionally, "Divided We Fall" was originally going to end with Superman revealing his secret identity to Lois at the end of the episode.[2]

Towards the end of the series, certain characters became off-limits to the show, namely any character associated with Batman (aside from Batman himself) and every Aquaman character. This was due to unrelated animated series The Batman, and the pilot of a Smallville-like series for Aquaman which wasn't picked up; as well as to not use any character from the Vertigo line. The decision to outlaw these characters is inconsistent with this theory, seeing as Smallville and Justice League Unlimited make use of several of Superman's supporting characters, namely Lex Luthor. However, this ruling would seem to apply to JLU's use of Lois Lane, as she did not have a speaking role once the character was added to the cast of Smallville. On the other hand, the sibling superheroes who are featured in the League, Hawk and Dove, are among the few Teen Titans who do not appear in the television series depiction of that superhero team. Conversely, Speedy was able to appear on JLU soon after Teen Titans ended, due to the length of the second season of JLU's production.

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.