The story begins in present-day Japan when the young Asumu Adachi witnesses the seasoned Oni, Hibiki, suffer a devastating defeat at the hands of a colossal sea-dwelling monster known as Orochi. As Hibiki lies gravely wounded, Asumu blames himself and desperately searches for a way to defeat the creature. His research leads him to an ancient document from the Warring States period, a text that astonishingly features his own name and tells of a time when humanity was terrorized by Orochi and its master, the Demon Network.

The narrative then shifts to the Sengoku era. A young boy named Asumu, a clear ancestor of the modern-day character, lives in a village that is forced to offer a yearly sacrifice to the monstrous Orochi. When a mark appears on his friend Hitoe, marking her as the next victim, Asumu refuses to accept her fate. Determined to fight back, he sets out to find Oni, the demon-fighting warriors who are both feared and shunned by superstitious villagers.

His quest leads him to a group of these warriors. He first encounters Kabuki, a charismatic but cynical Oni who agrees to help in exchange for rumored treasure. Together, they seek out other reclusive Oni across the war-torn land: the retired and guilt-ridden Hibiki, who carries the burden of his apprentice Takeshi’s death; the bored feudal lord Ibuki; the monk Touki; the cowardly merchant Kirameki; the imprisoned thief Nishiki; and the family man Habataki. They are later joined by the carpenter Todoroki, who reveals he was sent by the retired Hibiki.

As the seven warriors unite, tensions rise. The villagers, long terrified of both monsters and Oni, grow suspicious. After a group of villagers is murdered, a weapon belonging to Nishiki is found at the scene, turning the populace against them. The group fractures when it is revealed that Kabuki is the true traitor. Disillusioned by humanity’s rejection, Kabuki allied himself with the demons and framed his comrades. In the ensuing chaos, the demonic forces capture Hitoe, and the group of Oni disbands in despair.

In his darkest hour, the young Asumu discovers a katana forged by his late brother Takeshi, which was meant to be a gift for Hibiki. He brings the sword to the disgraced warrior, asking him to honor his brother’s memory. Reinvigorated, Hibiki dons his armor once more. As the other Oni, having reconsidered their abandonment, return to the battlefield, they unite to destroy Kabuki and his demonic army. In a final confrontation, they face the colossal Orochi itself.

The film returns to the present day, where the modern Asumu finishes reading the ancient text. He finds that the final page, describing the battle’s outcome, is missing. Sensing a connection, he rushes to a nearby cavern and discovers the very same katana from the Sengoku period, preserved across the centuries. He delivers the blade to a now-recovered Hibiki, who is once again fighting Orochi on a modern beach. Armed with the sword forged by his apprentice’s loyalty centuries ago, Hibiki transforms into his powerful Armed form and finally destroys the ancient demon for good. The story concludes by showing the past Asumu and the surviving Oni parting as friends, with the villagers vowing to create an organization to support the Oni in the future, a group they decide to name Takeshi in honor of Asumu’s fallen brother.