In the final years of the Heian period, a spirited princess named Ruri lives in the golden land of Hiraizumi with her father, a Minamoto clan leader, and her gentle older brother. Their peaceful existence is shattered when the powerful Heike clan attacks, burning down their mansion. Ruri escapes with only her loyal attendant Mitsumori, but they are soon cornered. Before them appears a man with piercing red eyes—a demon known as an oni. He offers Ruri a choice: give him her fate, her life force, in exchange for survival. Desperate to save Mitsumori, who has been mortally wounded, Ruri agrees, binding herself to the red-eyed oni named Akatsuki as his tsugai, a mystical partner. This bond restores Akatsuki’s power, and he protects Ruri from the Heike forces that pursue her. However, Ruri soon discovers that she is caught in a struggle among demons. Another oni, the Kurotsugai or black oni, also desires her as his tsugai, and he has allied with the Heike. Forced into a sham marriage with Akatsuki to secure her position, Ruri travels to the capital city, where the Kurotsugai lures her with a magical jewel. There, she confronts him and learns that both oni have their own designs on her blood and destiny. The story deepens when Ruri uncovers Akatsuki’s sealed past beneath his castle, Shiranui: a blue-eyed woman sleeping in an underground lake—Akatsuki’s former tsugai, who awakens and reasserts her claim on him. Ruri is thrust into a tangled web of love, vengeance, and supernatural politics as she fights to protect herself, her surviving loved ones, and discover the truth behind her own hidden nature.