A solar eclipse plunges the ancient capital of Heian into darkness, and as the shadow passes, a new terror emerges. Demons begin rampaging through the city at night, brutally attacking members of the nobility. Each victim is found with a specific body part savagely bitten off, leading the powerful onmyoji Abe no Seimei to suspect these are not random acts of violence but a ritualistic pattern. He theorizes that with the eighth murder, the seal on a legendary sword known as Ama no Murakumo will be broken, unleashing an apocalyptic age of darkness upon the empire.

The Emperor tasks Seimei with investigating the bloody incidents. He is joined by his close companion, the nobleman Minamoto no Hiromasa, whose loyalty and heartfelt nature often serve as a moral counterbalance to Seimei's more detached, mystical demeanor. Their inquiry leads them to Fujiwara no Yasumaro, the Right Minister, who fears for his daughter, Princess Himiko. She has begun sleepwalking at night, coinciding with the murders, suggesting a supernatural connection to the demonic entity.

As Seimei delves deeper, he uncovers a dark conspiracy that connects the Imperial Court to the Izumo tribe. The trail leads to a mysterious and powerful man named Genkaku, who has ingratiated himself with other court members by demonstrating god-like healing powers. Hiromasa, meanwhile, befriends a gentle peasant boy named Susa, who possesses a unique talent for playing the biwa lute and seems to have a mysterious connection to the unfolding events.

The narrative reveals a tragic backstory from eighteen years prior, when the previous Emperor ordered an attack on an Izumo village to plunder its sacred artifacts, including the sword Ama no Murakumo. Genkaku is revealed to be a survivor of that massacre who has sold his soul to a vengeful god. His plan is not merely political revenge but a ritual to resurrect the destructive deity Susano-o no Mikoto, using his own son Susa as a vessel. The eight body parts taken from the victims are the final ingredients needed to break the seal and complete the transformation.

The conflict escalates into a direct confrontation between the mysticism of the Heian court and the raw fury of a forgotten god. Himiko is revealed to be the reincarnation of the sun goddess Amaterasu, making her the final, crucial sacrifice in Genkaku’s scheme. As Genkaku succeeds in transforming Susa into the raging god Susano-o, the capital is ravaged. To counter this apocalyptic threat, Seimei must risk his own life, performing a dangerous ritual to resurrect the spirit of Amaterasu and restore balance. This leads to a final, desperate battle where the bonds of friendship and the struggle to stop a cycle of vengeance determine the fate of Heian-kyo.