The Justice Alliance is an intergalactic superheroes affiliation group who first appeared in it's Self-titled Episode.
In their first appearance, when they landed in Mega City, in search for hiring a new member into their group, ever since their long-lost friend, Captain Atomic, who used to be friends with their until he disappeared (based on their assumptions, but actually Mookie, who was responsible for turning Captain Atomic into a puppet). But however when Mookie came in to be a new member of the group, but was rejected a several times by several attempts, but were actually looking for Atomic Puppet, who refused to show himself while in this puppet form, while sadly looking through photos of himself and alongside with them in the past.
But later in the episode, as AP figured that Mookie was behind all this, when he figured that his friends was trapped by the green goo, and after saving their lair from being targeted into the sun, Cavalier was being questionable about the fate of Captain Atomic, and wasn't aware that AP is Captain Atomic, who Joey stated that Mookie transformed him into a puppet.
Then later decided to aceept AP's appearance as is and made AP and Joey, part of Justice Alliance Group.