Every 60 years, the heavens perform a sacred ritual known as ETM12. This tradition involves choosing worthy Eto-musume—celestial beings embodying various animals—to join the ranks of the Chinese zodiac, or Eto-shin. Yet, since the inaugural ETM12 two millennia ago, the original group of Eto-shin has ruled without any successors replacing them. Nyaa-tan is a cat Eto-musume who dreams of becoming a zodiac member during the current ETM12. Achieving this goal requires her to collect 12 seals, one from each Eto-shin. To do so, she must win diverse battles using Sol/Lull—divine energy generated by people's positive emotions. This challenge is formidable, as her abilities as an Eto-musume fall far short of those of even a single Eto-shin. Consequently, she requires a steady supply of energy. By chance, Nyaa-tan encounters Takeru Amato, a man who has just moved into the apartment where she is living in secret. To Nyaa-tan's joy, Takeru proves to be a source of high-quality Sol/Lull, distinguishing him from most others. Thus, the story of Takeru and Nyaa-tan unfolds. As Takeru aids Nyaa-tan in pursuing her dreams, he comes into contact with the Eto-shin and starts to reveal a mysterious past.