The eruption of Mt. Fuji in 1707 released hordes of demons from deep inside the earth. Sakuya, the young daughter of a samurai killed fighting these demons, accepts a mission to travel to Mt. Fuji and defeat the evil spirits. Accompanying her on her journey are two veteran warriors who served her father, and Tarō, a young kappa, or river spirit, whom she has adopted as her little brother. Along the way, the two warriors have doubts about Tarō’s loyalties, and the young kappa himself must decide if he will stand with his own species or with the humans who have cared for him.

While her husband is in prison doing time, Tamaki, the wife of a yakuza capo, runs her spouse’s gang with an iron hand. Meanwhile, Makoto, her younger sister, marries a member of a rival band after being raped by him. The two sisters, united by blood ties but married to enemy yakuzas, will ultimately have to decide whose side they’re on.

Set during the last days of the Tokugawa (Edo) Period, Moeyo-ken tells the story of Hijikata Toshizo and the Shinsengumi. Hijikata Toshizo is a samurai eager to test his skills and sword and make his first kill. The dojo he belongs to practices the Tennen Rishin style and teaches this to farmers. Not considered real samurai by other dojo's, they receive a request for a duel by a rival dojo practicing the Kogen Itto style. Sohaku Rokusha, a teacher there, challenges them and fights with Hijikata Toshizo. The outcome of the fight creates a deadly animosity between them that lasts through the ages.