Retsu Ukyo, a stoic Japanese surgeon in modern times, dedicates himself to medicine and a romance with Fuyuko Asahina. His imposing physique, reminiscent of Fist of the North Star characters—especially when wearing glasses and a white coat—stems from past experience as a college boxer. His demeanor remains unshaken even under extreme duress.

His life shifts dramatically when an elderly patient hires him to remove a malignant growth, which proves to be a parasitic demonic entity. The woman dies during surgery, exposing Retsu to an ancient conflict against demons originating 5,000 years ago in Japan's Jomon period. The supernatural Five Goddesses imbue him with psychic and physical powers, enhancing his combat abilities. These manifest as a luminous aura, though he often wields practical weapons like a hunting rifle.

Forced back to the Jomon period, Retsu confronts the demon civilization threatening humanity's future, aiming to stop their invasion at its source. He battles demons using his augmented powers and pragmatic tactics, such as targeting vulnerabilities like a demon's testicles.

Fuyuko Asahina becomes pivotal. After Retsu returns to the present, they share an intimate encounter offering mutual solace. This shatters when the Five Goddesses reveal Fuyuko is the demon queen in human guise, sent to seduce him. Despite her role as antagonist, she develops genuine affection for Retsu. Choosing to die as Fuyuko rather than her demonic self, she sacrifices her original identity. Their relationship potentially produced offspring, hinted by demon children buried in a temple, though unconfirmed.

Retsu's involvement inadvertently aids the demons; his union with Fuyuko provided biological material for breeding hybrid offspring designed for time travel. A temporal barrier prevents these hybrids from surviving the journey to the past. After the demon queen's defeat, unresolved threats linger, including the potential spread of demonic parasites in the modern era, which Retsu leaves unaddressed.