In a world where a strange disease known as Werewolf Syndrome exists as a chilling urban legend, those affected are said to transform and devour humans. Ten years before the story begins, Reito’s brother was killed by one of these werewolves. Driven by this tragedy, Reito has dedicated his life to working as a solitary detective, determined to uncover the truth about these monsters. He runs his agency, investigating acquaintances suspected of being werewolves, though society often views his work as little more than occult consultations. Reito is a cautious and prepared man, always carrying a silver knife that can immobilize a werewolf, and he keeps a low profile, believing that his appearance becoming public would hinder his investigations.

The narrative takes a sharp turn when Reito is reunited with his nephew, Yoru, whom he has not seen in a decade. Yoru, once a crybaby, has grown into a tall and handsome university student who towers over his uncle. After Yoru reveals he has been following Reito, he expresses his desire to work as a part-time assistant at the detective agency. Reito is conflicted; he wants to protect Yoru, whose parents were also killed by werewolves, but he suspects that a werewolf has been stalking him and believes keeping Yoru close is dangerous. Despite his reservations, Reito agrees to let Yoru visit.

Tensions rise as Reito observes Yoru’s strange behavior, particularly when he seems to avoid tasting human secretions, a known trigger for werewolf transformation. Suspicion turns to near-certainty when Reito finds that Yoru’s phone screensaver is a photo of him taken before their supposed chance reunion, along with a note containing his home and office addresses, confirming that their meeting was not a coincidence. On a night when the werewolf instincts are strongest, Reito prepares for an attack. He finds Yoru standing over a journalist who had been helping him with his werewolf research. Confronted, Yoru admits his deception and that he has been exposed. However, instead of devouring Reito as expected, Yoru leans in and kisses him, revealing a far more complex and personal objective than simply feeding.