Master ninja Hiro Hattori is trying to sleep when he has an unexpected visit from his friend Kazu, a fellow shinobi and member of the same ninja clan working undercover at the shogunate. Kazu says that Saburo, the Shogun's cousin, has been stabbed to death within the walls of the Shogun's palace with Kazu's dagger, and that though he is innocent, he fears he will be blamed for the murder. He begs Hiro's help in escaping the city. But before he can flee, Hiro and Father Mateo, the Jesuit priest under Hiro's protection, are summoned to the palace to aid in the investigation. There they learn of a plot to assassinate the Shogun and overthrow the ruling Ashikaga clan. With Lord Oda, a rival warlord, scheduled to arrive in Kyoto soon and the murderer poised to strike again, Hiro must use his assassin's skills to reveal the killer's identity and protect the Shogun. Kazu, trapped in the city, still will not admit where he was at the time of the murder, and this makes Hiro doubt his innocence. Other suspects include the maid who found the body, Saburo's wife, and the stable master.
Unfortunately, the Shogun demands the murder solved before Lord Oda arrives, and if the murderer can't be brought to justice, Hiro and Father Mateo may have to die in his place.