Every 60 years, the heavens conduct a sacred ritual called ETM12. This custom involves selecting worthy Eto-musume—celestial beings representing different animals—to become one of the members of the Chinese zodiac, or Eto-shin. However, since the first ETM12 two thousand years ago, the original batch of Eto-shin reigns with no one being able to replace them.

Nyaa-tan is a cat Eto-musume who aspires to become a member of the zodiac in the ongoing ETM12. Fulfilling her ambition requires her to secure 12 seals, one for each Eto-shin. To that end, she must win various types of battles using Sol/Lull—divine energy created by people's positive emotions. This task is not easy however, as her powers as an Eto-musume are far below the abilities of a single Eto-shin. As such, she needs a constant source of energy.

But in a chance encounter, Nyaa-tan meets Takeru Amato, a man who has just transferred to the apartment where she is secretly staying. To Nyaa-tan's delight, Takeru discovers that he gives out high quality Sol/Lull—something that sets him apart from most people. With this, the story of Takeru and Nyaa-tan begins. As Takeru supports Nyaa-tan in her dreams, he meets the Eto-shin and begins to uncover a mysterious past.