Vampire Knight tells the story of Yuki Cross, a student at the prestigious Cross Academy, which her adoptive father runs. She and her adoptive brother/best friend Zero are members of the Disciplinary Committee whose main job is to keep the Day Class students away from the Night Class students since these ones have a secret: they are all Vampires, and it's imperative that stays a secret.
Yuki knows a little about them, when she was just a small child, the leader of the Night Class, Kaname, saved her from a rough Vampire, that's her first memory. Kaname was the one who brought her to Mr. Cross when he discovered she had no memories.
Zero also knows about Vampires, but he has no liking for them; he and his family were hunters before a vampiress caught with them and killed everyone else- including Zero's twin -but him.
There are many secrets in this story and it's sequels, this is just the first part, and I have to be honest and say that when I started it I though the first few episodes were SO boring. But I stuck to it, mostly because I was intrigued by Zero, and the story does pick up toward the end.
The design of the characters and the series over all is beautiful, and that's the first thing that drew me to it, in the end, I found it to be a worthy series, and lots of people seem to really love it - I only liked it but I'm Vampire prejudiced. By far I think Zero is the best character around, and learning his story was what made this series for me.