Homeless and fresh out of employment, Mito Yamamoto faces familiar loneliness and despair. Luck is simply not on Mito's side; when he stumbles upon a cafe and cuts his hand, a strange server named Ruka Saotome licks the blood from his wound and even dramatically retches at its apparent disgusting taste. Offended and dispirited, Mito runs off to a bridge, hoping to put an end to his misery.
But when Mito accidentally slips, he is miraculously saved by Ruka, who reveals he is a vampire. Unable to drink blood from just any woman for mysterious reasons, Ruka proposes that Mito become his thrall—a vampire's human servant and primary source of food. In this deal, Ruka will shower Mito with love to sweeten his blood, and Mito will gain a warm place to call home. However, there is an issue: Mito is a girl! Concealing her long, flowing hair and feminine body, Mito readily takes this chance and enrolls in the Hijirigaoka Boys' High School while working part-time at the cafe with Ruka. But the more Mito's bond with Ruka deepens, the more she struggles to hide her secret.