Tatsuki and Ohta are two beings from a supernatural, mystical realm masquerading as "cousins" in the human world. Tatsuki is in fact the successor to Ryu-Oh, the dragon king, and possesses the power of the Nyoi-Houshu, a magical dagger. Ohta is a Zashiki Warashi, a protective house-spirit without a home and is also Tatsuki's subservient lover. When their human classmate Koushi Misaki accidentally witness the two having sex, he mistakenly believes that Tatsuki is forcing the quiet Ohta into it, and he resolves to break the two apart. Tatsuki and Ohta have escaped to the human realm to avoid Tatsuki's marriage, which would have to take place if he is to become the new ruler. Even when he learns that Ryu-Oh is dying, Tatsuki refuses to return to his own realm. But when Ohta is gravely injured in a car accident, the only way to save him is to return to the home world. There, Ohta recovers, and it is discovered that his power of purification is able to restore the purity of Ryu-Oh's Houshu (treasure-sphere, the source of his magical power), thus bringing Ryu-oh back to health.