A young girl named Haru awakens to another day, where it seems she's a few steps behind everything. Rushing to school, she comes in late for class, to mocking laughter from her classmates, and even from one guy in her class whom she has a crush on.
After school, walking home with her friend Hiromi, Haru curses her misfortune. As they walk down a busy street, they see a dark cat carrying a small box in his mouth. In the middle of the street, the cat drops it, and attempts to pick it up, unaware that a large truck is heading towards it! Haru grabs her friend's lacrosse stick, and running into the street, saves the cat, before collapsing into some bushes and breaking the lacrosse stick. Unseen by anyone but Haru, the cat then stands on it's hind legs, and thanks her for saving him. It then gets back on all fours, and rushes off. When Haru relates the incident to Hiromi, her friend can scarcely believe it either.
Later that evenin, Haru tells her Mother about the incident. Her Mother explains that it sounds like something that happened when Haru was little. Supposedly, a dirty white kitten was following her as she ate fish crackers. Haru then took pity on the kitten, and gave her the whole box. Haru's Mother claims that Haru told her that the kitten talked to her, but Haru doesn't recall anything like that happening.
Later that night, Haru hears something outside, and sees a procession of cats walking down the street. Then eventually stop outside Haru's house, and she comes outside. Among the procession are the Cat King, his Prime Minister Natori, and a servant cat named Natoru. Natori explains that the Cat King thanks Haru for saving his son, and she is presented with a scroll, that gives information about numerous gifts she will receive for her good deed.
The next day, Haru wakes up, wondering if it was a dream. Her ponderings are soon put to rest when a number of things happen. Her friend Hiromi has awoken to find numerous lacrosse sticks outside her door. At Haru's place, cat tail plants surround their house, and as she heads to school, a number of cats chase after her (Haru surmises that she must have some catnip on her). At school, she is freaked out when she finds a number of individually-boxed mice in her shoe locker.
Getting over the shock, she goes about the day, taking Hiromi's clean-up duty upon her request. While taking out the garbage, she sees the guy she has a crush on, walking by with his girlfriend. This distracts Haru and she trips and falls, feeling more miserable than the day before. Suddenly, she sees the King's assistant, Natoru, who becomes shocked when Haru yells at him regarding the 'gifts' she has received. Noting that she seems upset, Natoru offers to take her to the Cat Kingdom for a personal tour, and even tells her of the additional gift she will receive for her good deed: the chance to marry the Cat King's son, Prince Lune. This shocks Haru, who then starts to wonder if being among cats wouldn't be so bad, since they seem to have very few responsibilities. Natoru takes this as a 'yes' answer, and rushes off, leaving Haru distraught at the idea of her (a human girl) marrying a cat! As she panics, a disembodied voice tells her to find a white cat at the crossroads, who will lead her to the Cat Bureau.
Haru goes to the specific location, where she finds a large white cat. The cat doesn't say a word, but she follows him through all sorts of little nooks and crannies, until he reaches a small embankment of shops that almost seem doll-like in size. Here, the large white cat gets up on his hind legs, and heads to a small storefront. As he does so, the setting sun catches on the windows surrounding them. The front door of the building opens, and a cat in a suit emerges, giving his name as Baron Humbert von Gikkingen, but Haru decides to just call him 'Baron.' Baron cites the large white cat as Muta, and as they talk, a stone statue comes to life, which Baron says is named Toto.
Baron invites his guests into his home, where Haru explains about her predicament, and how a strange voice told her to seek out the Cat Bureau. The Baron decides that he and Muta will make a visit to the Cat Kingdom in regards to Haru's request for their help. However, the Baron requests that Haru never forget who she is, as this will be very important. As they speak, there is a knock at the front door. Haru answers it, only to find Natoru there, and a number of cats who carry Haru off, intending to take her to the Cat Kingdom. Muta manages to catch up with them, but Baron and Toto lose their trail.
Haru wakes up in the Cat Kingdom, where she finds she has shrunk to the size of a cat. Looking around, she is met by a white cat named Yuki, who pleads with Haru to leave the Cat Kingdom at once. However, Natoru appears, and takes both Haru and Muta to the King's palace.
Haru is taken to be dressed to meet the King, while Muta is escorted to a banquest hall, where he gorges on a number of foods. As Haru changes, she is then met by the King, who is overjoyed that she has chosen to marry his son. However, Haru explains her trepidations, since she is a human, and the Prince is a cat. The King and Natori smile, telling her that that is no longer a problem, since Haru is now half-cat. Haru looks in a nearby mirror, and is shocked to see that she has somehow turned into a cat! Running from the room, she finds Muta, stuck in a bowl of catnip jelly, unable to move.
Even though Haru is still upset over being turned into a cat, she is taken to the main dining room, where the King attempts to cheer her up with a number of entertainment acts. When none seem to please Haru, he has the performers tossed out a nearby window. Suddenly, a masked strange comes forward, promising to cheer up Haru. As the band strikes up a waltz, Haru dances with the mysterious stranger, feeling more at ease being a cat. This feeling causes her to grow whiskers, which makes her gasp. The masked stranger then tells her not to forget who she really is, as he said so before. Suddenly, the Cat King calls for the music to stop, demanding to know who the stranger is. He removes his mask to reveal the Baron! The Cat King sends his troops to take down the Baron and retrieve Haru, but this quickly fails. In the ensuing scuffle, Muta is freed from the bowl of catnip jelly, and joins the battle.
Off to one side, Yuki appears and hastens Baron and Haru to her. She explains that there is a way for Haru to regain her human form, and helps her and the Baron escape through a secret entrance, which leads to a maze outside the castle, of which a large tower extends from it's center. Yuki explains that if they can get to the top before a specific time, they will be able to return to their world.
Muta then joins them, and they set out going through the maze. However, the King sends a number of troops, and tries to throw them off course. His plans fail, and Haru, Baron and Muta make it to the tower, climbing up it's outer stairway. Just when it seems they've made it halfway-up, the King detonates the tower, causing the mid-point to plummet to the bottom!
Haru, Baron and Muta are then met by the King, though the reunion is short-lived when Prince Lune arrives, along with a contingency of soldiers. The King explains to the Prince about Haru, but the Prince explains that he wishes to marry Yuki. He then hands her the same box that Haru saw him with on the street. The box contains some fish crackers. Yuki then explains that they are just like the kind that Haru gave her when she was a kitten, and thanks her for helping her all those years ago.
The King finds this very moving, but then tries to see if Haru will marry him instead. Haru angrily rejects his advances, and Muta happily proclaims that he admires her spirit.
The King, not one to be spurned, still tries to claim Haru, but Muta and Lune's soldiers prevent him from advancing, and Haru heads up the nearby steps for the top of the tower. Baron also confronts the King, and wins in a duel against him, before hearing Haru cry out from the top of the tower...which leads to the other world, but whose opening is several hundred miles up in the sky! Muta and Baron rush to her aid, but not before she falls over the edge.
They freefall towards Haru's hometown, when a flock of crows appears, courtesy of Toto. Haru makes it safely down, and they say their goodbyes, with Haru admitting that she has a bit of a crush on Baron, before he takes off.
The next day, Haru's Mother awakes, and is shocked to see that Haru is up before her, having already prepared breakfast. Later on, Haru meets up with Hiromi, who happily tells her friend that the guy she has a crush on has broken up with his girlfriend. However, instead of being excited at this, Haru shows no concern, as this is now unimportant to her. It appears that her experience has allowed her to become more independent.