Taro lives on Neoterror, a planet 12.5 light years from Earth, where artificial humans have reconstructed Earth’s civilization within a peaceful society mirroring Earth’s past. As their descendant, his existence reflects 250 years of planetary stability. His innate personality is introverted and defined by reluctance toward action or initiative.

A pivotal shift occurs when Taro finds a mysterious life form named Tamager hidden within purchased eggs. This encounter grants him the power to transform into "Gainaman," a super-positive superhero persona starkly contrasting his usual demeanor. As Gainaman, he undertakes missions to resolve incidents across the city, actively solving problems and protecting others.

His sister Hanako gains similar transformation abilities through Tamager, becoming Gainaman No. 2 and aiding in these activities. Simultaneously, agents from the AI management bureau pursue Tamager, driving conflict tied to the life form’s origins and purpose. The tension between Taro’s reserved nature and his assertive, heroic role as Gainaman forms a core element of his journey.