Fuyuko Asahina begins as the romantic partner of surgeon Ukyo Rettsu, actively supporting his investigation into Japan's historical ties to an impending demonic invasion. She joins him in exploring ancient sites, including an old temple and Jomon period ruins, as they search for answers about the prophesied conflict.

Her story twists when she is unmasked as the demon queen in human guise—an identity adopted after her demonic civilization fell 5,000 years earlier. Throughout history, she pursued relationships with human men to spawn demon offspring and rebuild her race, though the supernatural Five Goddesses repeatedly thwarted her efforts.

After the apparent resolution of the demon threat in the past, Asahina urges Ukyo to join her in death, confessing genuine love. When he refuses, she reveals her true demonic form and attacks him. During the ensuing battle, aided by the Five Goddesses, Ukyo defeats her. With her dying breath, she reaffirms her love for him.