In a dystopian version of Japan, humans and vampires converge on a lawless district known as the Blood Night Market, or Blood Night City. This red-light district is a chaotic bazaar where the currency is human blood; vampires come to drink, and desperate humans come to sell their own life essence. Within this dark metropolis, the value of a human life is determined solely by the quality and taste of their blood.

The protagonist, Sotetsu, is a young human man who works for the Blood Control Bureau, a government organization tasked with policing illegal vampire activities. He carries a heavy burden: he is branded with the mark of "wasted blood" or "undesirable blood". Because his blood is deemed unpalatable, no vampire has ever desired to drink from him, leaving him isolated and worthless in the eyes of the very creatures he polices.

Sotetsu’s monotonous life of enforcing regulations is upended when he encounters a mysterious and flamboyant vampire named Mozu. Mozu is a self-proclaimed "gourmand" or "food scholar" who claims to be a connoisseur of unique blood types. Contrary to every other vampire, Mozu becomes intensely fixated on Sotetsu, declaring that his "worthless" blood is actually a rare delicacy that he craves.

The series follows the volatile and dangerous relationship that forms between the two. Mozu forcibly inserts himself into Sotetsu's life, even joining the Blood Control Bureau to become Sotetsu's partner. However, his obsession is possessive and violent. Early in the narrative, Mozu attacks Sotetsu and, after a battle with a rogue vampire, forces the injured human into a predatory "contract" that resembles bottle-keeping—reserving Sotetsu exclusively as his personal feeding stock. This bond blurs the line between partner and prey, placing Sotetsu in a living hell where he is physically dominated and possessed by the very monster he fears.

The story moves beyond their initial violent courtship to unravel larger conspiracies. A criminal organization known as the Farmer begins to move shadows in the Blood Night Market, instigating conflict. Simultaneously, Sotetsu begins to uncover the secrets of his past, including the mystery surrounding his mother's death. He also discovers hidden truths buried in Mozu’s long history, specifically relating to the "Ambrosia," a mythical or legendary blood source that drives vampire desires. As Sotetsu tries to maintain his humanity and his job, Mozu fights against the Blood Control Bureau’s regulations to keep Sotetsu isolated and exclusively his. The narrative concludes with a final confrontation where the intertwined fates of the lonely human and the egotistical vampire reach a resolution, exposing all secrets and determining whether their bond is one of salvation or mutual destruction.