“You must not drink blood until you are an adult!” Muroii Yoshikage is a diligent vampire with a love for cleanliness, about to enter high school, but without many friends. One day, he meets up with someone he got to know online—a "slightly" crazy girl named Serika—but not two meetings later, she has him tied up and asks him to drink her blood?! But Yoshikage lives by his family's rules, one of which is, "You must not drink blood until you are an adult!" True to his principles, he refuses Serika's attempts to seduce him, but will he be able to keep it up for long...?