In the year 1614, the retired shogun Tokugawa Ieyasu faces a succession crisis over which of his grandsons will become the third Tokugawa shogun. To settle the matter, he revokes a long-standing non-aggression pact between two rival ninja clans, the Koga of Manjidani and the Iga of Tsubagakure, and commands each to select ten elite warriors to fight to the death. The clan that eliminates all ten of the opposing side wins, and the grandson they represent will be named the undisputed heir.

The Koga and Iga clans have harbored a bitter feud for four centuries, and their members possess extraordinary, often inhuman abilities developed through generations of selective breeding. Among the chosen combatants are the two young heirs: Koga Gennosuke, grandson of the Koga leader, and Iga Oboro, granddaughter of the Iga leader. Despite their clans’ hatred, Gennosuke and Oboro have fallen deeply in love and were betrothed in the hope that their union would finally end the ancient bloodshed.

Now they are forced into opposing sides of a brutal conflict neither wants. Gennosuke wields the basilisk eye, a dojutsu that turns an attacker’s killing intent back upon themselves. Oboro possesses the occult-piercing eye, which neutralizes any ninja technique or ability directed at her. Their personal tragedy mirrors the larger slaughter as the twenty ninja unleash their unique arts—spider-like agility, shape-shifting, poisonous breath, near-immortality, and more—in a series of lethal encounters across the Japanese countryside.

The narrative follows the relentless elimination of the combatants, each death narrowing the path toward an inevitable confrontation between the two lovers. The story explores themes of duty versus love, the cost of hereditary hatred, and the manipulation of warriors by political powers beyond their control. Major arcs include the initial surprise attack by the Iga, the Koga’s journey to confront Ieyasu and the second Hattori Hanzō, and the final tragic stand between Gennosuke and Oboro, where their personal bond is tested against the weight of their clans’ destinies.