Ace is one of four robot dogs assisting the protagonist, Tsutomu, in the fight against an alien threat. The dogs were originally ordinary strays befriended by Tsutomu. They suffered mortal injuries from an explosive, cactus-like bomb deployed by the alien Devil Seijin while protecting Tsutomu and his father, Professor Hayashi. Using their surviving brains, Professor Hayashi rebuilt them into powerful cyborgs for his son to command.

Ace's cybernetic abilities include a left eye equipped with night vision, x-ray vision, and a caloric tracking camera, and a right eye that functions as a plasma gun. He also possesses the capability of flight.

The four cybernetic dogs can combine into a single, larger form, transforming into a jet spacecraft called the Liner, which Tsutomu pilots. In this combined state, they employ a tracking beam fired from the jet's nose and can execute ramming attacks. The unit can also separate into individual jets to fight in Battle Formation N.2, with each dog-jet firing red lasers from their noses.

Ace operates under Tsutomu's direct command; using a special helmet, Tsutomu can understand the dogs' language and issue orders. Their primary mission is to combat the Devil Seijin, a parasitic alien race seeking to colonize Earth and eliminate its native population from their secret base inside Mount Fuji.