Mahiru Shirota follows the adage of "less is more," avoiding trouble as much as possible. After school, he shows kindness by taking in a kitten and naming it Kuro. Little does he know that this one selfless act will greatly impact his humdrum life. One evening, Mahiru returns home to a shocking sight: a strange young man in his house. Panicked, he exposes the stranger to sunlight, only to watch in astonishment as the man transforms into Kuro. Kuro is a lazy, reclusive vampire who promises to depart at dusk. However, an unfortunate chain of events leads Mahiru into an accidental agreement with Kuro, thrusting him into the treacherous realm of supernatural beings and bloodthirsty creatures. Mahiru's life is no longer simple.