In 1535, Obayashi Kansuke, a masterless samurai, is on his way home to succeed his adoptive father after 15 years of journey studying military strategies. At a deserted village he drops in at, he saves a girl from a samurai. The samurai is one of the soldiers under Takeda Nobutora, the warlord of Kai, who are in the battle with the Imagawa clan of Suruga. The girl named Mitsu falls in love with Kansuke. Meanwhile, the Hojo family of Sagami invades the domain of Kai. Kansuke goes to watch the battle between the Takeda and Hojo armies. Though Kansuke has stopped her, Mitsu follows him and is caught by the soldier of the Takeda clan again.

Kansuke returns home in Mikawa Domain after 15 years. He brings the decapitated head of the samurai of the Takeda army he killed to save Mitsu—he have planned to show it as a display of his prowess in order to be taken into the service of the Imagawa clan. Coming back, Kansuke finds that his adoptive father now has a natural son. The father who wants the son, not Kansuke, to succeed him steals the head and tenders it to one of the warlords under the Imagawa clan as the son’s achievement. Kansuke decides to rename himself Yamamoto Kansuke, the family name of his real parents, and leaves Mikawa for Suruga, where his was born. In Suruga, Kansuke happens to know his elder brother Sadahisa is involved in a conspiracy against the head of the Imagawa family. For knowing the secret, Kansuke is attacked by a group of assassins directed by Sadahisa. Kansuke has to leave home again.

Kansuke goes back to the Kai domain, where he meets Mitsu. She is expecting a baby, whose father, she insists, is Kansuke. Though he refuses her once, Kansuke gradually becomes attached to the unaffected nature of the villagers and eventually makes up his mind to live there with Mitsu and the baby she is carrying. Meanwhile, the ruler Nobutora and his young son and heir Harunobu are on bad term. Nobutora wages wars one after another, provoking complaints against him among his retainers as well as common people. One day, Nobutora goes hunting to get nothing. Irritated, he shoots an arrow at a human—the woman aimed at is Mitsu.

Mitsu has been shot and killed by Nobutora. Kansuke keeps calm, but hearing Takeda’s retainers impose silence on the villagers about what happened, Kansuke attacks in fury at Nobukata Itagaki, one of Harunobu’s chief retainers. Though Kansuke is defeated, Itagaki gets impressed by his swordsmanship and tells Harunobu about the one-eyed man. Harunobu, who is also shocked by the cruel behavior of Nobutora, wants to see Kansuke. The two meet each other for the first time. Sensing Kansuke’s resentment against Nobutora, Harunobu persuades him to lay aside his grudge and hold ambition. Kansuke goes to Suruga on Itagaki’s order to spy into the Imagawa clan at internal strife. Kansuke informs Tadatane Ihara, his relative who is one of Imagawa’s main retainers, of the conspiracy against the clan leader. Ihara, however, tells Kansuke that the Imagawa leader as well as his brother has died mysteriously on the same day.

Kansuke is introduced to Jukeini, the mother of a potential successor to the late head of the Imagawa clan. Kansuke proposes the woman who has tremendous influence in the clan a plot to dispose of Fukushima, the betrayer who secretly communicates with Takeda Nobutora. Having no way to know that Jukeini’s son is working on amicable settlement with the Takada clan, Kansuke plans to take revenge on Nobutora for Mitsu when the head of the Takeda family comes to help Fukushima. Fukushima takes up arms against the Imagawa family but Takeda’s army does not turn up. Kansuke’s brother Sadahisa, who has been working under Fukushima, kills himself before Kansuke’s eyes.