Cross Academy is attended by two groups of students: the Day Class and the Night Class. At twilight, when the students of the Day Class return to their dorm, they cross paths with the Night Class on their way to school. Yuki Cross and Zero Kiryu are the Guardians of the school, protecting the Day Class from the Academy's dark secret: the Night Class is full of vampires!

Outside the safe boundary of the academy, Yuki is attacked by a vampire. Injured and unable to defend herself, two Night Class students come to her aid and kill the vampire. When Yuki asks them why they killed their own kind, they tell her to come to the Moon Dormitory at midnight to learn the answer...