In 1638, in the aftermath of the Shimabara Rebellion, the swordsman Miyamoto Musashi and Yui Shōsetsu witness an impossible event: the dead rebel leader Amakusa Shirō is resurrected from within a woman's body through a dark ritual. The ritual is the work of Mori Sōiken, a sinister old man who served the defeated warlord Konishi Yukinaga at the Battle of Sekigahara. Sōiken has developed a forbidden technique called Ninpō Makai Tenshō, or the demon-world reincarnation technique. By modifying a woman into a special vessel known as a shinobi-body and having a dying swordsman copulate with her, the swordsman is reborn one month later in a healthy, demonic form, twisted into a cruel and violent creature. Sōiken's ambition is to use this power to overthrow the Tokugawa shogunate.

Shōsetsu immediately becomes Sōiken's disciple, and together they begin seeking out legendary swordsmen on the verge of death, offering them rebirth. Over the years, one after another falls to the temptation: the vengeful fencer Tanmiya Bōtarō, the master swordsman Araki Mataemon, the invincible warrior Miyamoto Musashi, the stern Yagyū Tajima-no-kami Munenori, the spear saint Hōzōin Inshun, and the elder Yagyū Nyōunsai. They become the Seven Demon Men of the Makai Tenshō. Sōiken and Shōsetsu then approach Tokugawa Yorinori, the lord of Kishū, and fan his ambition to seize the shogunate. Yorinori agrees and begins preparing for rebellion, using the seven demon swordsmen as his strike force.

The eighth recruit Sōiken desires is Yorinori's own son, the swordsman of the Yagyū clan, Yagyū Jūbei Mitsuyoshi. Jūbei is a carefree and roguish wanderer, but he is also a peerless swordsman. Sōiken sends the kunoichi Clara Ojō to seduce him and turn him to their side, but the plan fails. Meanwhile, Yorinori begins collecting young women as vessels for more reincarnations. Among them are Ojō, Oyotsu, and Ohina, three beautiful daughters of the Kishū clan's top retainers who have come to admire Jūbei. When the retainers discover the demon swordsmen are slaughtering the women, they send the girls to safety with Jūbei and are themselves killed. The dying messenger reveals that the enemy includes revived swordsmen, but he keeps secret the identities of Munenori and Nyōunsai. The demon swordsmen approach Jūbei but, under orders to recruit him, withdraw.

Jūbei devises a plan. He goes to the Kishū mansion, claims to be a spy for the shogunate official Matsudaira Izu-no-kami, and declares that he and his ten followers, the Yagyū Ten People, will make a pilgrimage circuit through Kishū territory to fight the demon swordsmen. Amakusa Shirō, representing Sōiken, accepts the challenge. The demons see it as an opportunity to test Jūbei and also intend to defeat him one by one. Shirō sends Ojō as a vessel to infiltrate Jūbei's group.

The pilgrimage becomes a series of deadly duels. At the first temple, Seiganto-ji, Jūbei kills Tanmiya Bōtarō. At Sandanbeki Cliffs, he kills Hōzōin Inshun with the help of one of his men. Ojō is discovered but Jūbei spares her, sensing her innate kindness. A frustrated Yorinori, hearing that Shōgun Iemitsu is gravely ill, begins acting on his own. At Dōjō-ji, Yagyū Nyōunsai ambushes Jūbei. Jūbei wounds Nyōunsai's eye but is forced to retreat when Nyōunsai takes Ohina hostage. Jūbei then follows Ojō into the enemy headquarters at Wakayama Castle, where he faces Nyōunsai again. With Ojō's secret help, Jūbei kills Nyōunsai but fails to rescue Ohina. Ojō is betrayed and seized by Shirō. Jūbei arrives at the temple Kii-dera and fights Shirō in the rain. With Ojō's aid, Jūbei kills Shirō, but Ojō takes a fatal shuriken meant for Jūbei. As she dies, she destroys one of Sōiken's magical fingers, preventing further reincarnation. Shirō, also dying, reveals that the final, secret demon is Jūbei's own father, Yagyū Tajima-no-kami Munenori.

Yorinori, furious at his losses, marches toward Edo with a thousand men, passing through Yagyū territory. Munenori, now a demon, uses hostages to draw Jūbei out. Jūbei challenges his father to a duel at the Hōtoku-ji temple and defeats him by exploiting Munenori's obsession with a scroll. However, Jūbei's men break the terms of the duel, and the hostages are taken away by Mataemon. Yorinori enters the empty Yagyū castle and meets Sōiken, who reveals that Iemitsu's illness was a ruse by the shogunate to lure the traitor. The shogunate official is nearby, and they send Mataemon to scout. Jūbei intercepts Mataemon at Keya-no-Tsuji and kills him in a duel after a bullet cracks his blade.

The shogunate official, Matsudaira Izu-no-kami, enters Yagyū castle alone and persuades Yorinori to abandon his rebellion. Sōiken, enraged, goads Yorinori to commit one final reincarnation using one of the captured women. Before the ritual can happen, Musashi, the remaining demon, decides to betray Sōiken and Yorinori, intending to offer their heads to the shogunate for a position. He kills Sōiken and flees. Jūbei rescues the hostages and threatens Yorinori, forcing him to return to Kishū. Jūbei then pursues Musashi, luring him to a small island off Kuwana, reminiscent of the legendary Ganryū-jima. There, the two swordsmen face each other. Musashi waits with a long wooden sword, and Jūbei predicts he will use it because of his memory of the duel with Sasaki Kojirō. With a single vertical slash, Jūbei cuts through the wooden sword and kills the last demon. The story ends with Jūbei kneeling before Musashi's body, hands clasped in prayer.