影片改编自同名漫画，故事以住在澡堂打工的吸血鬼·森兰丸（吉泽亮 饰）和天真无邪的少年·立野李仁（板垣李光人 饰）为中心展开。兰丸追求极致的味道“18岁处男的血液”，目睹了澡堂的独子李仁的成长和纯洁。然而有一天，李仁对同学篠冢葵（原菜乃华 饰）一见钟情。于是兰丸的必死“阻止童贞丧失作战”拉开帷幕。

Ranmaru is a 450-year-old vampire who works in a bathhouse. As a vampire he is particularly picky about his blood choices, his main goal being to drink the blood of an 18-year-old virgin. To do so, he has been watching 15-year-old Rihito, the son of the bathhouse owner who saved him from near death. Rihito must not lose his virginity before he turns 18 and Ranmaru does everything he can to prevent that. However, when Rihito starts to show interest in a classmate, Ranmaru's desperate fight to preserve the boys virginity begins.