A school split into two classes: the Day Class, normal humans attending normal classes, and the Night Class: a group of beautiful men and women who take classes at night...and are secretly vampires.
Founded by headmaster Kaien Cross, a former vampire hunter, the school is an attempt to prove that humans and vampires can live in peace. This has been possible primarily due to an understanding between the headmaster and Kaname Clan, with Kaname taking charge of a group of young vampires and bringing them to the school. For the most part things are peaceful for the Day Class, who remain unaware of the true nature of the Night Class.
During the day, the Day Class--identified by their black uniforms--have their normal classes. At twilight the Day Class is sent back to their dorm and the Night Class moves from their dorm to the school building. To ensure the peace between the classes, two prefects exist who are aware of the Night Class's identity: Yuuki Cross and Zero Kiryuu. They oversee the twilight change in classes, and run night patrols to ensure that no Day Class students are trying to sneak into the Night Class--and to make sure the Night Class isn't running loose on the campus.