The prestigious private school, Cross Academy, consists of two sessions, the Day Class and Night Class. Although the Night Class consists of students that are all beautiful, intelligent elites, they are in fact vampires! As members of the Disciplinary Committee, Yuki Cross and her childhood friend Zero Kiryu keep a close watch over the students of the Day Class, who are infatuated with the Night Class students, while acting as appointed Guardians for the Night Class to protect their true identity.

Yuki is busy patrolling the school grounds for Day Class girls who are trying to sneak chocolates to the Night Class boys for St. Xocolati's Day.

When Zero suddenly leaves the school grounds, Yuki goes after him and roams the streets alone, until a mother and child appear before her.