Haru Yoshioka is the protagonist of this story, an ordinary high school girl navigating the difficulties of adolescent life. She lives with her mother, Naoko, as her father is absent and not mentioned in the narrative. At the beginning of the story, Haru is portrayed as insecure, awkward, and somewhat unlucky. She struggles with punctuality, often arriving late to school, and exhibits a general clumsiness, such as tripping over her own feet. Her lack of organization and a wishy-washy attitude contribute to her feeling like an outsider amongst her peers, including her crush, a boy named Machida who barely notices her.

Despite her insecurities, Haru possesses a core of kindness and a cooperative spirit that endears her to both humans and animals. Her love for cats is a defining trait, rooted in a childhood memory where she shared a cookie with a stray kitten named Yuki. This act of kindness may be the reason she can understand the speech of cats. Her compassion is impulsive and active; she risks her own safety by running into traffic with a lacrosse stick to save a strange cat from being run over. This single good deed, however, sets off an unexpected chain of events.

The cat she saves is Prince Lune of the Cat Kingdom. As thanks for rescuing him, the Cat King begins sending Haru gifts from a feline perspective, including a field of catnip and boxes full of live mice, which quickly turn her life into a chaotic mess. The situation escalates when the king decides the ultimate reward is for Haru to marry the prince. Haru's inability to firmly and directly refuse the king's envoy is her fatal flaw; her flustered silence is misinterpreted as consent, leading to her being forcibly taken to the Cat Kingdom.

Once there, Haru's role shifts from a passive victim to an active agent in her own rescue. She seeks out the Baron, a suave and gentlemanly cat figurine who runs the Cat Business Office, and his rotund companion Muta. Her key motivation becomes a desperate desire to return to her human life and her former self, before she loses her identity completely. As she remains in the Cat Kingdom, she begins a slow transformation into a cat, starting with shrinking in size, growing ears and a tail, and developing whiskers. This physical change mirrors her internal fear of losing herself to passivity and the whims of others. Throughout her escape, she builds crucial relationships. The Baron acts as her guide and protector, encouraging her to believe in herself, while Yuki, the cat she fed as a child, reveals herself as a lady-in-waiting in the Cat Kingdom and repays Haru's long-ago kindness by helping her escape.

Her journey leads to significant character development and a symbolic transformation. Tired of being pushed around by the Cat King, Haru finally stands up for herself, rejecting the marriage and declaring her desire to be herself. Upon her successful return to the human world, she has physically changed, having cut her long hair short. This important haircut represents her newfound confidence, maturity, and resolve. She loses her previous obsession with being popular, moving on from her crush on Machida, and even admits to having developed a small crush on the Baron, indicating she now values character over superficial coolness. By the end of the story, the once clumsy and uncertain girl has become bold and self-assured, learning to trust her own instincts and appreciate her own worth. Her notable ability is her capacity to understand cat language, a gift that enables her entire adventure.