Justice League Unlimited

The episode starts with Brainiac 5 and Bouncing Boy of the Legion of Super-Heroes in an abandoned headquarters. Bouncing Boy complains about running away from a fight and "Brainy" explains that they need to regroup to fight later, to which Bouncing Boy replies that since only the two of them are left it's useless to fight. Brainy then uncovers a time machine as their solution. The two Legionnaires then brood over the fact that, according to the incomplete historical records, only two out of the three heroes they will bring from the past will ever return…

Supergirl is training with Green Lantern on a hologram "danger room" while Superman and Green Arrow look on. Green Arrow remarks that Supergirl looks up to Superman but Superman doesn't even want to spend time with her on her 21st birthday, and Green Lantern and Green Arrow seem to have been more family for her than Superman. It seems that she changed her costume to look more like Superman's. Kara beats up her training foes with flying colors and Green Lantern says he has nothing left to teach her, and Green Arrow comes up and congratulates her on setting a new record time.

Just then a sphere of energy surrounds them and Superman arrives at the scene to find them gone...

In the future Bouncing Boy and Brainy welcome the abducted heroes and after a little misunderstanding regarding Brainy's heritage they start talking. The heroes agree to help while Bouncing Boy warns that Supergirl is the one fated to die on this very day. Two members of the villainous Fatal Five show up and the heroes get thrashed. Everybody gets mind control disks on them but Brainy, a mega-genius, uses his willpower to fight his. He then frees Supergirl who frees Green Arrow. The Emerald Empress and the Persuader leave with a mind-controlled Green Lantern and Bouncing Boy.

Green Arrow sees a budding love between Brainy and Kara. After pointing out to Brainiac 5 that he is in love, he leaves them to it while going off to upgrade his arrows. With Kara's aid, Brainiac 5 manages to track the Fatal Five through the JLU comlink and the three go off to the Central Capitol Planet where the enemy is headed. A ship moves to intercept them and dozens of mind-controlled Legionnaires and Green Lantern emerge to attack them. Supergirl proposes to take on all the mind-controlled Legionnaires and Green Lantern single-handedly while the other two take out the mind control machine on board the enemy ship. As she prepares to go, Brainiac 5 tells Supergirl of her predestined fate to never return to her time because of her death in the future. Brainiac 5 and Supergirl kiss and go on their respective missions.

Brainy uses "the direct approach" and crashes into the Fatal Five's ship. Supergirl uses her speed to get the jump on Green Lantern – she sends him flying and starts swatting all the other Legionnaires, easily outmatching their strength. Cosmic Boy tries wrapping metal around her but she uses heat vision to escape. Chameleon Boy becomes an alien creature and attacks her alone and she spins him off her. Three blaster-type heroes fire at her but she beats them up too. The mind-controlled Green Lantern reenters the battle and gets the advantage on her, applying a crushing vise.

Brainiac 5 and Green Arrow hold their own long enough to knock out the mind control device, saving Supergirl in the nick of time. Kara and company then rush to save the outnumbered Brainy and Green Arrow.

While the Legion says goodbye to the JLU "capes," Kara says she is going to stay in the future and have a relationship with a Brainiac about five generations younger than her. Green Lantern and Green Arrow return to give Clark a hologram message from Kara, and Superman regrets that he never told her he was proud of her. Green Arrow tells him not to worry – she knows. Then Superman asks the name of the boy Kara mentioned and Green Lantern and Green Arrow look uncomfortable…

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The Legion of Super-Heroes

The Legion first appeared in Adventures Comics #247 (April 1958) and were created by Otto Binder and Al Plastino. The three founding members, Lightning Boy (later Lightning Lad), Cosmic Boy, and Saturn Girl traveled from the 30th century to visit Superboy in the 20th century and offered him membership since he was their historical inspiration. Intended as a one-shot, they became so popular that they guest starred again and later got their "own" comic in Adventure Comics (sub-titled "Superboy and the Legion of Super-Heroes"). Supergirl was also inducted as a member, and both her and Superboy traveled to the future to participate in Legion adventures. They went through several different back-up features until finally getting their own comic in January 1980.

The Legion requires that each member have at least one certifiable "super power" unique from any other member's. Some members have "powers" shared by those of other members of their home planets, while others have acquired their power through accidents or experimentation. There have been several exceptions: Superboy and Supergirl have the same powers, as did the 20th century hero Mon-El who later turned up in the 30th century after being banned to the Phantom Zone. The non-powered Karate Kid was inducted after demonstrating that his mastery of the martial arts was sufficient to hold Superboy at bay. The Legion has had literally dozens of members at any one time, making it arguably the largest superhero group at any given time, not counting sidegroups like the Legion of Substitute Heroes, the Reservists, the Honorary members, and the Legion of Super Pets (don't ask).

The Legion has gone through several "reboots" due to the alteration of history (and the on-again/off-again existence of a Superboy in the 20th century), cloning experiments, etc., and as of the time this episode aired they are on their fifth series.

In the WB animated universe, the Legion first appeared in the Superman: The Animated Series episode "New Kids in Town", when Cosmic Boy and Saturn Girl (and Chameleon Boy, replacing Lightning Lad from the original comic) visit a teenaged Clark Kent in Smallville and thwart Brainiac's plans to alter history.

The Fatal Five

The Fatal Five first appeared in Adventure Comics #352 (Jan. 1967), and were created by Jim Shooter and Curt Swan. Undermanned and faced with the thread of the Sun Eater, five Legionnaires were forced to bring together the five most wanted criminals of the future. The villains aided them in return for a pardon, but discovered it was more profitable to remain together as a team. They returned to plague the Legion on many occasions.

The Five's core membership consists of: their leader, Tharok, a petty criminal who had half his body vaporized in an explosion while fleeing the police. His missing half was replaced with a cybernetic construct that boosted his genius to superhuman levels and built-in weaponry and the ability to control machinery. Validus, the unknown child of Saturn Girl and Lightning Lad, stolen by Darkseid and augmented into a giant behemoth with strength to rival Superboy's and the ability to proejct blasts of brain lightning. The Emerald Empress, Sarya of Venegar, the deposed ruler of her home world, she found an ancient Oan artifact with powers similar to a Green Lantern's power ring but none of its weaknesses. The Persuader, an unnamed resident of a heavy-gravity planet who wields an "Atomic Axe" of unknown origins which can cut through any form of matter or energy. And Mano, a mutant born with a distintegrator disk on his right hand and disfigured features. When his people rejected him, he built a spacesuit containing his planet's toxic atmosphere and then destroyed the planet.

As with the Legion, the Fatal Five's history has changed throughout the various reboots, and they've briefly added a few other members, but these five members in whatever incarnation have always been the core members. This is their first appearance in the WB animated universe.

Brainiac 5

Pre-Crisis, Brainiac 5 first appeared in Action Comics #276 (May 1961) and introduced himself as Querl Dox of the planet Colu. He applied to the Legion of Super-Heroes as a super-genius wishing to atone for the sins of his ancestor, the original Brainiac of the 20th century. It was later revealed that he was a descendent of the Coluan who led the rebellion against Brainiac, who called himself Brainiac 2. Brainiac 5 used his genius to aid the Legion in a number of ways, creating their Legion flight rings, a cure for Mon-El's lead poisoning, and designing a nearly-impenetrable personal force field belt. His genius also led to occasional bouts of insanity and he had an infatuation with Supergirl who traveled from the 20th century to fight with the Legion.

A reboot or two later after the original Crisis and Zero Hour, Brainiac 5's origin was rewritten somewhat to reflect the post-Crisis Brainiac. He has gone through bouts of computer-link non-emotion and currently he is the descendent of Vril Dox of L.E.G.I.O.N.

In the WB animated universe, Brainiac 5 first appeared briefly in the Superman: The Animated Series episode "New Kids in Town". In all of his incarnations, Brainiac 5 is a twelfth-level genius with some martial arts training and a force-field belt but no other superhuman abilities. This is his first significant appearance in the WB animated universe.

Bouncing Boy

Bouncing Boy first appeared in Action Comics #276 (May 1961) where he applied for membership along with Brainiac 5 and others. Charles (Chuck) Taine was a delivery boy who accidentally drank an experimental serum a sporting match and gained the superhuman ability to inflate and bounce. No, really. Due to the artificial nature of his powers his abilities faded from time to time but were restored. He eventually retired from active Legion duty and became a Legion Reservist and an instructor at the Legion Academy. He married fellow Legionnaire Duo Damsel. Post-crisis, he serves as the Legion's architect and engineer and has no superhuman abilities. Originally Bouncing Boy had the ability to inflate himself and bounce, as well as compute angles, ricochets, and velocity with greater-than-normal ability, but had no other superhuman abilities. This is his first appearance in the WB animated universe.