A young girl named Haru finds herself struggling through her days, constantly feeling a step behind everyone else. Despite her best efforts, she arrives late to class, only to be met with the mocking laughter of her classmates, including a boy she has a crush on. After school, while walking with her friend Hiromi, Haru complains about her misfortunes. As they stroll down a bustling street, they encounter a mysterious dark cat carrying a small box. In a moment of urgency, Haru rushes into the street to save the cat from an oncoming truck, but in the process, she collapses into some bushes, breaking Hiromi’s lacrosse stick. Miraculously, the cat, who remains unseen by anyone else, stands on its hind legs and thanks Haru before scurrying away.
When Haru shares this bizarre incident with her mother, her mother recounts a memory from Haru’s childhood, about a dirty white kitten that once followed her while she was eating fish crackers. Although Haru’s mother insists she once claimed that the kitten spoke to her, Haru cannot recall any such conversation. That night, curious sounds draw Haru outside, where she discovers a procession of cats led by the Cat King, along with his Prime Minister Natori and a servant named Natoru. They express gratitude for Haru saving the King's son, presenting her with a scroll that outlines the various gifts she will receive for her good deed.
The next morning, bewildered and questioning the reality of last night’s events, Haru finds an array of items outside her house, including numerous lacrosse sticks at Hiromi's doorstep. As she makes her way to school, a swarm of cats inexplicably follows her, and her locker is filled with individually-boxed mice, escalating her confusion. Despite all this, Haru tries to carry on normally. When she spots her crush with his girlfriend, it distracts her, leading to another unfortunate fall. Shortly after, she encounters Natoru, who expresses surprise at her reaction towards the gifts she's received. Noticing her distress, Natoru offers to take Haru to the Cat Kingdom, revealing a shocking additional gift: the opportunity to marry the Cat King’s son, Prince Lune. This proposition leaves Haru in utter disbelief, leading her to contemplate the carefree cat life, devoid of responsibilities. However, this only exacerbates her panic when a mysterious voice mentions finding a white cat at the crossroads to guide her to the Cat Bureau.
Following the directions, Haru finds the large white cat, but it remains silent as it guides her through enchanting little shops. Eventually, they reach a storefront, where Baron, a cat dressed in a suit, greets them. Baron introduces the large white cat as Muta, and they engage in conversation. Haru shares her predicament, prompting Baron to visit the Cat Kingdom with Muta on her behalf, but he impresses upon her the importance of remembering her identity.
When Natoru and a group of cats arrive to escort Haru to the Cat Kingdom, Muta slips in to join her, although Baron and Toto cannot follow. Arriving in the Cat Kingdom, Haru is shocked to discover that she has been transformed into a cat herself. Yuki, a white cat, implores Haru to flee the kingdom, but Natoru interjects, bringing them both to the King's palace. While Muta indulges in a lavish buffet, Haru is dressed up to meet the King, who exuberantly celebrates her impending marriage to Prince Lune. However, Haru’s concerns escalate when she realizes she is no longer fully human. As the King claims her, the masked stranger from earlier reappears, assuring Haru to maintain her true self.
With mounting tensions, the Cat King’s troops clash with Baron and Muta, leading to an escape for Haru and Baron, thanks to Yuki’s guidance through a secret exit that could potentially lead them back home. Racing against time, they navigate a maze to reach the tower's top. Just when they think they've made it, the menacing King detonates the structure, sending them tumbling into chaos. Upon arriving at the top, they confront Prince Lune, who reveals his love for Yuki instead. Amidst the confusion, Haru asserts herself, and with Muta's unwavering spirit alongside her, the battle rages on as they attempt to escape and return to their world.
In a suspenseful series of events, Haru finds herself plummeting from the tower but is saved by a flock of crows led by Toto, ultimately landing safely back in her hometown. In the aftermath of these surreal events, Haru’s perception of life changes radically—she wakes before her mother, makes breakfast, and later meets Hiromi. Though her friend shares encouraging news about Haru's crush, Haru's newfound independence renders such worries trivial, illustrating her growth from her extraordinary adventure in the Cat Kingdom.
