In the Ichinose household, three sisters and their mother live together. The oldest, Hinano, is pretty, friendly, and easygoing. However, she's also rather messy and lazy in her personal life. The middle sister, Ichino, is stern and easy to anger, but she's incredibly hardworking and does a lot of the chores and cooking around the house. The littlest sister, Satsukino, is a lovable little girl who adores animals and wants to be a vet when she grows up. Together, they live with Kyouno, their extremely strict, though caring, mother.

One day, Satsukino takes in a big, fat, talking cat. After the three sisters convince their mother to let them keep it, they name it Tonnura-san. But what is this mysterious cat, and where does it come from? How will the lives of the Ichinose girls change now that they've accepted Tonnura-san into their family?