During the Sengoku period, a young boy named Kotaro Yuri Chinen escapes unknown pursuers with his loyal dog Tobimaru. The monk Shouan guides him to seek help from Master Zekkai at the Mangaku Temple in the Akaike Province. Meanwhile, a Ming force, led by the elderly master Bai-Luan, is escorted by Lord Akaike's soldiers who have agreed to allow the Ming to build a grand altar on his land in exchange for gold. The convoy is ambushed by bandits, but the Ming are brutally slaughtered by Luo-Lang, the Ming's famed Western swordsman.
As Kotaro and Tobimaru seek shelter in an abandoned temple, they encounter Nanashi, Tomoya Nagase, a wandering swordsman. A Ming warrior attacks, and Nanashi kills him, though Tobimaru is wounded by a poisonous dagger. To protect the boy, Kotaro hires Nanashi as a bodyguard, and Nanashi escorts Tobimaru to a doctor for healing. Back in Akaike, Tu-Si, the second Ming envoy, is captured and tortured to reveal the purpose behind the altar. Tu-Si eventually discloses the Ming Emperor’s plan to create the Xian Medicine, an immortality elixir that requires the blood of a prophesied child—Kotaro—who can only be sacrificed at a specific time of year. Hearing this, Lord Akaike shifts his strategy to seize Kotaro for a steep ransom.
Nanashi shares a fragment of his mysterious past with Kotaro, explaining that he has fought for various masters and knows little about his own origins beyond being a shipwreck survivor with red hair he once dyed black to blend in. He leaves Kotaro in the care of Shouan and the Mangaku monks, but the monks betray them in a bid to save themselves and surrender Kotaro to the Ming. When Akaike forces finally close in, Nanashi senses the betrayal and returns, only to find that the Ming have already taken Kotaro. Tobimaru leads Nanashi through pursuit, tracing the captors to a fortress guarding the altar.
Bai-Luan’s betrayal reaches a grim climax as the Ming seize him and use him as a human shield for the fortress siege. Itadori Itadori, an ambitious Akaike general, leads a small battalion to rescue Lord Akaike but ultimately seizes the opportunity to turn on his own commander, plunging the troops into a bloody clash. The fighting rages on, and Itadori falls as the fortress burns around them.
Nanashi arrives at the fortress and, for a moment, is knocked unconscious. He recalls a pivotal memory: years earlier, he was ordered to execute two children and vowed never to unsheathe his sword again. When he regains consciousness, he finds Kotaro poised to be sacrificed at the altar. Nanashi draws his blade and fights his way through the defenders to reach the altar and save Kotaro. Bai-Luan’s treachery is revealed once more as Luo-Lang kills him, and the two swordsmen—Nanashi and Luo-Lang—engage in a blazing duel that tears apart the structure itself. Despite grievous injuries, Nanashi defeats Luo-Lang, who dies with a look of stunned respect.
At dawn, Kotaro rides away on horseback with Tobimaru, carrying the badly wounded Nanashi, and speaks of starting a new life together. The journey hints at a quiet, uncertain future for the trio, bound by loyalty, sacrifice, and the chance of a fresh beginning beyond the shadow of their perilous past.
