In the world only the aristocrats have the power to fight the monster - mana.
Even though Melida Angel was born in an aristocracy and studies at an academy developing mana users, she has no mana. To find her talent for mana, Kufa Vampir was sent to be her tutor but at the same time, he was ordered to assassinate her if she has no talent for mana. He makes a cruel decision to assassinate her because he thinks that it is a wasted effort that people who have not any mana are working hard in this world... However, he asks her, "Do you want to try to entrust your life to me?" He is not as an assassin nor a tutor. He is someone betting on his pride as an assassin tutor, to show the world what Melida is capable of!