Jack, one of four stray dogs—Queen (the mother), Ace, and Joker—befriended by Tsutomu Hayashi, begins as an ordinary canine. The dogs exhibit fierce loyalty confronting an alien threat: Queen detects a bomb disguised as a cactus planted by Planet Devil invaders and sacrifices herself by carrying it from the Hayashi home, assisted by Ace, Jack, and Joker, resulting in their destruction.

Professor Hayashi, Tsutomu’s father, preserves their brains and transplants them into mechanical bodies, transforming them into cyborgs. Jack’s enhancements specifically grant him fire-breathing capabilities. Alongside Queen, Ace, and Joker, he joins a combat unit defending Earth against insectoid aliens. The four cyborg dogs combine with Tsutomu’s command to form the spaceship "Liner," Earth’s primary defense.

Throughout the conflict, Jack participates in direct assaults against the aliens, deploying his fire-breathing during battles on Earth and the final invasion of Planet Devil. His actions help thwart the invaders, including neutralizing the acid-spitting snail monster Escargon and preventing the detonation of a Moon Bomb targeting Earth.