This is the basic plot...
Yuki Cross's earliest memory is of being attacked by a vampire and saved by Kaname Kuran, a pureblood vampire for whom Yuki tries to deal with her seemingly unobtainable love for her hero. Ten years later, Yuki, along with her childhood friend, Zero Kiryuu are school guardians (prefects) at Cross Academy where her adoptive father, Kaien Cross is the headmaster. Their jobs as guardians is to protect the human "Day Class" students from the vampire "Night Class" students and ensure that the human students do not discover the secret of the Night Class. The story follows Yuki's discovery that Zero had been turned into a vampire and her struggle to keep him sane, to prevent his condition from deteriorating into an Level E vampire. Yuki attempts to give up her humanity and be turned into a vampire by Shizuka Hio, Zero's family's murderer and his master, in a bargain for Zero's sanity. Zero stops Yuki, preferring to kill Shizuka. Neither occurs when Zero's assumed dead twin (Ichiru Kiryuu) shows up, and Kaname kills Shizuka and drinks her blood. Zero's grip on sanity slides and Yuki continues to help Zero fight his bloodlust while also seeking to recover her lost childhood memories suspecting Kaname knows more than he lets on. Kaname instead gives Zero his own blood to help keep Zero from going insane.