In feudal Japan, Kai is a half-Japanese, half-British outcast who lives in the village of Ako, ruled by the benevolent Lord Asano Naganori, who once found a young Kai lost in the woods and accepted him into his domain. Despite being rejected by the samurai led by Oishi due to his ancestry, Kai becomes a skilled warrior and falls in love with Asano's daughter, Mika.
Ahead of the arrival of Shogun Tokugawa Tsunayoshi, Ako is visited by his master of ceremonies, Lord Kira, who becomes attracted to Mika. He arranges a fight between his best soldier, the Lovecraftian Samurai, and one of Oishi's men, Yasuno, who is later found under the spell of Kira's adviser, the witch Mizuki. Kai takes his place and fights the Lovecraftian Samurai, but is defeated and humiliated. Later that night, Mizuki bewitches Asano into believing Kira is raping Mika, causing Asano to attack an unarmed Kira, an offense punishable by death. The Shogun allows Asano to die honorably by committing seppuku, and brands Oishi and his men Ronin. Kira demands to marry Mika, but the Shogun grants her one year to mourn the death of her father, and forbids the Ronin from seeking revenge. To ensure this, Kira traps Oishi in a well in an attempt to break his spirit.
Nearly a year later, Oishi is released by Lord Kira's men. He reunites with his family and asks his son Chikara to reunite the Ronin, including Kai, who had been sold into slavery, to avenge Asano and save Mika. After the local blacksmiths deny the Ronin help out of loyalty to Kira, Kai leads them to the Tengu Forest, a mystical place he once escaped from as a child. Kai instructs Oishi to never draw his sword while in the Tengu temple and continues alone to another room. While Kai fights the Tengu Master, Oishi watches his men fighting the Tengu and dying. Oishi follows Kai's advice to never draw his sword and the illusion disappears; having earned the respect of the Tengu Master, the Ronin are given swords. Armed with their new weapons, the Ronin plan to attack Kira on his pilgrimage to prayer for his wedding. Instead, the group is ambushed by Kira's forces, led by Mizuki and the Lovecraftian Samurai. Several of the Ronin are killed, and Mizuki, believing to have slaughtered them all, takes Oishi's sword and presents it to Lord Kira as a trophy. Mizuki later taunts Mika with her death and attempts to manipulate Mika into killing herself.
Kai and Oishi, having survived the attack, reunite the remaining survivors to fulfill their mission. With help of a band of traveling performers, Kai and Oishi infiltrate Kira and Mika's wedding, giving Mika an opportunity to flee. Meanwhile, the Ronin climb the walls and fight Kira's men, including the Lovecraftian Samurai, who is killed by a bomb. Kai and Mika attempt to escape together, but are attacked by Mizuki, who takes the form of a giant serpent, but is slain by Kai. Meanwhile, Oishi and Kira fight, and Oishi finally beheads Kira, avenging his lord.
The 47 Ronin are sentenced to death for seeking vengeance against the Shogun's orders. Again, the Shogun allows them commit seppuku to preserve their honor. Impressed by their bravery, the Shogun allows Chikara to live in order to preserve Oishi's bloodline. After the deaths of the Ronin, Mika finds a letter written by Kai, in which he promises to find her once she crosses into the "other world". A scroll with the blood of the 47 Ronin is preserved as a symbol of their unwavering honor, strength and dedication to justice.