After Musashi rescues Michiru Saruwatari from a demon, his friend Kojirou Kanemaki discovers that his father was a member of the Uesugi clan. Determined to learn more about his own past, Kojirou—alongside his Kanemaki clansmen—decides to participate in Uesugi's punitive expedition against the demon lord of Awajima Island: Yamata no Orochi, an eight-headed dragon that threatens to engulf the entire region.

However, upon joining Uesugi's military alliance, Musashi faces the harsh reality of fighting against powerful demons. Humiliated by his lack of power and inability to work in groups, he has to learn how to cooperate with his newfound allies. Meanwhile, the Obsidian Eight—a group pursuing the Obsidian Goddess that dwells within Musashi's body—resumes its hunt for Musashi, and plans to thwart the ongoing island operation. As the situation grows chaotic, Musashi and his friends must foil the machinations of the various factions conspiring to seize the Obsidian Goddess.