Feeling bored with the routine of daily life, high schooler Haru Yoshioka finds excitement when she saves Prince Lune of the Cat Kingdom from a dangerous accident. As a gesture of thanks, the Cat King showers her with gifts and invites her to become Lune's wife. Despite her struggle to connect with her feline counterparts, Haru misunderstands the situation and becomes trapped in the situation. When a mysterious voice tells her to find the Cat Bureau, Haru begins her quest to escape, but gets abducted by a group of cats on the way. The owner of the Cat Bureau, Baron Humbert von Gikkingen, follows them to the Cat Kingdom out of concern for their safety. As Haru grows more involved in the world of cats, she begins to transform, becoming more cat-like with each passing day. Distressed at the realization that she may never return to her human form, Haru embarks on a journey of self-discovery, learning more about herself than she ever could have before. When she eventually returns to her normal life, Haru is a more confident person with a deeper sense of self-awareness.