In 1604, Miyamoto Musashi attacked the Yoshioka family at their dojo and defeated master Seijūrō and his younger brother Denshichirō in two duels. To save their reputation, the Yoshioka family decides to fight back with all 100 family members and hire an additional 300 samurai. Now Musashi sets out to defeat all 400 enemies in his most famous battle.

Haru, a schoolgirl bored by her ordinary routine, saves the life of an unusual cat and suddenly her world is transformed beyond anything she ever imagined. The Cat King rewards her good deed with a flurry of presents, including a very shocking proposal of marriage to his son! Haru embarks on an unexpected journey to the Kingdom of Cats where her eyes are opened to a whole other world.