Ooga Miko has arrived in the city to look for her older sister. While wandering around, she saves the life of a cat at the expense of her own. When Miko awakes, the cat from before is there, and she talks and transforms into a human girl!
The cat is named Nekomiya Niu, and in return for Miko saving her, she gave her one of her cat lives. Niu attends Mamori Middle School, which is overseen by the Varied Lifestyles Administration Bureau, who looks out for animals blending into human society. The school’s chief director informs them that Niu’s cat life only amounted to giving Miko one more year to live. In order to regain a full life, Miko must now help Niu’s sister Miu with solving 100 cases! Thus begins Miko’s strange adventures with transforming animals!