Robo-kun is a discarded home-appliance robot inhabiting a junkyard. Designed to create artificial rainbows by sprinkling water, his core function becomes secondary to an aspirational goal: becoming a justice-fighting robot, inspired by classic mecha anime. This ambition drives him to undertake various heroic endeavors despite his small size and physical limitations.

His attempts to achieve hero status lead to multiple initiatives with unintended outcomes. He accidentally thwarts a robbery, then strives for a record-breaking high-altitude head jump. He pursues heavy construction work, ending in dismissal and unintended city-wide risks. He obsessively attempts to create rainbows during relentless rainfall. Inspired by television coverage, he participates in an atmospheric launch, inadvertently demonstrating the ease of rainbow-making.

A significant development occurs when he encounters another robot marked with the kanji for "evil" (悪) on its chest. This robot excels at tasks Robo-kun struggles with, prompting him to question his own usefulness and purpose. Despite this rivalry, they collaborate on seasonal gestures, such as preparing a Christmas surprise for their human companion.

Robo-kun's final heroic effort involves a large-scale battle, fulfilling his long-held mecha-inspired fantasy. This culminates in an implied revelation concerning his background and relationship with the girl in the junkyard, suggesting a deeper pre-series connection that contextualizes their dynamic. His character arc revolves around persistent optimism amid repeated failures, a growing understanding of his capabilities versus aspirations, and his evolving bond with his companions.