Mahiru Shirota is a high school student who likes simple things and dislikes complicated things, so he picked up a cat that he named Kuro. The next day, he finds out that Kuro is actually a vampire who only metamorphoses into a cat if exposed to sunlight. Mahiru accidentally makes a contract with Kuro the moment he calls his given name in his human form. Kuro explains that he is a special type of vampire who makes a contract and then serves humans. He can only drink blood from his master and must listen to their orders...