Every sixty years, a sacred celestial event known as ETM12 takes place. This ritual selects worthy Eto-musume, divine beings representing different animals, to join the ranks of the Chinese zodiac, called Eto-shin. However, since the tradition began two thousand years ago, the original Eto-shin have never been replaced, preventing any new members from entering.
Nyaa-tan, a feline Eto-musume, dreams of becoming part of the zodiac during the upcoming ETM12. To achieve this, she must collect twelve seals from each Eto-shin, which requires her to engage in various battles fueled by Sol/Lull—a divine energy drawn from human positive emotions. Unfortunately, Nyaa-tan's power as an Eto-musume is far weaker than that of an Eto-shin, so she needs a constant supply of this energy.
By chance, Nyaa-tan encounters Takeru Amato, a new resident of the apartment complex where she secretly lives. Takeru unexpectedly produces high-quality Sol/Lull, setting him apart from ordinary people. As Nyaa-tan works toward her goal with Takeru's help, he also gets to know the Eto-shin and slowly discovers a hidden past.