Joker is one of four former stray dogs, befriended by Professor Hayashi's son Tsutomu, who were killed by insectoid aliens from Planet Devil. Professor Hayashi resurrected the canines as cybernetic entities. Joker possesses specialized combat functions, including the ability to transform and combine with the other three cyberdogs—Queen, Ace, and Jack—forming the composite spaceship "Liner." This merged unit engages in interstellar combat against the alien invaders.

Assigned to Earth's defensive unit alongside Tsutomu, Joker operates as part of this integrated team. The cyberdogs' abilities adapt contextually, deploying specific weapons or skills as tactical situations demand during confrontations. The narrative emphasizes their collective function as a singular protective entity. Joker's role remains confined to this operational purpose within the core storyline.