When retail worker Kosaka Haru sees a man attack another on the street, he thinks very little before intervening. As he wakes up the next day in an abandoned building surrounded by a ravenous mob, he wonders what he has gotten himself into. Soon, Akihito, the man he saved, arrives and informs Haru that he had been mortally wounded in the attack. The only way Akihito could have saved Haru was to bite him—and turn him into a vampire. Vampire transformations are not immediate, and Haru now exists as a "tank," a partially-converted human, for the foreseeable future. Tanks are considered an irresistible reservoir of blood to other vampires, as biting one of these individuals does not have the consequence of creating offspring. Akihito promises to protect Haru during this perilous stage, but his feelings toward Haru—and Haru's feelings toward his own vanishing humanity—remain uncertain. [Written by MAL Rewrite]