The story takes place towards the end of the 20th century and explores the idea that the human world secretly coexists with the demon world with a secret police force known as the Black Guard protecting the boundary.
The existence of the "Black World" is known to very few people. For centuries, a pact between the two has been observed to maintain peace, and terms must be negotiated and renewed every few hundred years to continue relative harmony. This time around, there is a militant faction called The Radicals that will stop at nothing to prevent the signing of a new treaty.
Two agents of the Black Guards are charged with insuring the success of the treaty: The human Taki Renzaburo is an electronics salesman by day, and a Black Guard agent when needed; his partner Makie, who masquerades as a model, is a beautiful and skilled woman from the Black World. Their mission is to protect Giuseppi Mayart, a two hundred year-old man possessed of fantastic spiritual power, whose presence at the peace treaty signing is critical. The Radicals wish to kill Mayart to upset the peace between both worlds.
Attacks on Makie, Taki and Mayart begin even before the three meet, and the situation does not improve, despite taking shelter in a hotel that supposedly has strong spiritual barriers to keep people of the Black World away; on top of this, Mayart sneaks out after a skirmish at the hotel. Makie and Taki find him at a massage parlor in the grip of a Black World woman who has sapped his health, prompting a frantic trip to a spiritual hospital under Black Guard protection.
Halfway there, Makie is taken prisoner to be punished for her "crimes" against the Black World, and Taki is forced to leave her behind, but as soon as he knows Mayart is safe in the hospital, he rushes to where his partner is being held, despite all warnings and the threat that he will be thrown out of the Black Guard.
While Taki is successful in freeing Makie, they are captured by a spider-like woman Taki has encountered before; both are knocked unconscious, but they wake up alone in a church, and seek comfort in each other's bodies.
One last attack by the Radicals comes and is partially deflected by a surprisingly healthy Mayart, who reveals he was protecting his bodyguards, not the other way around as they had been lead to believe. Mayart and Taki almost succeed in defeating the Radical threatening them, but the final blow comes from Makie, who suddenly displays an overwhelming power, a gift from her joining with Taki. Mayart explains the two of them are meant to be part of the new peace treaty.
The movie ends with Taki reinstated in the Black Guard, uncertain about his feelings for Makie and what is expected of them, but feeling optimistic about the future he will help protect.