A decade ago, 450-year-old vampire Ranmaru Mori was saved from the brink of death by Rihito Tatsuno, a young and innocent boy. Ever since then, Ranmaru has been working at the Tatsuno family's bathhouse with a secret: he wants a taste of Rihito's virgin blood as soon as he turns 18. However, his plan is threatened when Rihito falls in love with a girl on his first day of high school.

Jealous and scared that Rihito's pureness will be ruined, Ranmaru begins to lurk in the shadows to protect him from any harm. During his operation, the vampire must keep his real identity hidden and make sure his target's virginity remains unsullied.