Bloody is a vampire unlike any other that Ichirou Rosario di Maria has met. He has red hair and red eyes, refuses to drink blood or hurt humans, and cannot die even when exposed to sunlight. The two meet when Bloody rescues Ichirou from a group of vampires who desire the coveted "Blood of Maria" running through his veins—its delicious scent lures in vampires like a siren call, which has caused Ichirou recurring problems. After defeating the low-level nuisances, Bloody begs Ichirou, whom he calls "Maria," to finally end his interminable life.

At first, Ichirou is skeptical that Bloody is as harmless as he seems, and does not know why he would be able to successfully kill a seemingly immortal vampire. As it turns out, Bloody has him confused with another "Maria"—Isaac Rosario di Maria—a famed priest and exorcist who lived four hundred years ago. Ichirou inherited the cross-shaped rosario that his grandfather carried, and should possess the same ability to exorcise even the strongest vampires—such as Bloody—but is unfortunately unaware how to call upon his dormant powers.

Determined to survive the repeated attacks on himself, Ichirou makes a deal with Bloody, who he renames "Mary"—until the day comes that Ichirou can fulfill Mary's death wish, the vampire must do everything he can to protect this single human.