Makoto Okazaki is an unpopular high school student who is frequently bullied by some of his classmates, being forced to act as their errand boy. But his life drastically changes one night when he is attacked by a mysterious girl, who plunges her teeth into his neck and begins drinking his blood. Upon satisfying her craving, she offers Makoto a simple choice: become like her or die.

When he wakes up in a hospital after his encounter, Makoto is unable to deal with the blindingly bright lights and finds that water cannot quench his growing thirst.