A busy corporate worker named Kiriko dies and is reborn in a fantasy world where a permanent rainbow arcs across the sky. In this new life she appears as a nine-year-old girl and discovers she has the power of a beast mage. Shortly after arriving, she is rescued by a gigantic Pomeranian who bears an uncanny resemblance to Potechi, the Pomeranian she had loved and lost in her previous life. Overjoyed, Kiriko names the creature Potechi and begins traveling on its fluffy back.

Kiriko soon learns that her beloved cat Pino has also been reborn somewhere in this world. Determined to reunite with Pino, she sets off on a journey across the rainbow-lit landscape. Along the way she befriends a beastman named Togi, who joins her party, and encounters various other fluffy animals and kind-hearted people. Her beast magic steadily improves as she faces obstacles and helps those she meets.

The journey takes a tense turn when Kiriko meets a suspicious beast tamer named Lauri, who commands a black cat identical to Pino. Kiriko manages to rescue Pino using her magic and also breaks a curse that had afflicted Togi. But a new crisis emerges: every time Potechi uses his power, he shrinks smaller and smaller. To restore Potechi’s strength, the party heads toward the legendary Rainbow Valley. There, the truth behind Potechi’s divine nature and the reason for the eternal rainbow are revealed. In the end, Kiriko’s love and the bonds she has formed allow her to save Potechi, bringing the story to a gentle, tearful conclusion.