Shiro, a white dog whose Japanese name reflects his fur color, was discovered as a puppy inside a cardboard box by a young boy and joined the boy's family. Now five years old—sharing the boy's birth year—he appears as an average household pet by day. Secretly, he transforms into the superhero Super Shiro via a special badge, gaining heightened physical prowess and a deeper, heroic internal monologue while his barks remain ordinary.

Highly intelligent and capable of logic exceeding humans, Shiro independently walks himself if forgotten, forages efficiently during irregular meals, and boards public transport by posing as a plush toy. He fiercely protects the family's baby daughter, repeatedly risking himself for her safety, and performs trained tricks like scratching or curling into a cotton-ball-like shape.

His covert superhero mission is to protect Earth by locating the legendary Bobobobobone—a bone said to confer world domination—identified by its "glorious, elegant, and wild" scent. Potential finds are teleported to his alien ally Bibo for authentication; failures detonate, restarting the quest. His rivals include Dekapoo, a power-hungry inventor dog, and Can Can, a social-climbing female dog seeking the bone.

Despite occasional family neglect, Shiro stays loyal, inhabiting a doghouse frequently commandeered as the boy's hideout. He activates his superhero identity only when family members are distracted or away.