Shirota Mahiru is someone that loves simple things and hate complicated ones, but he can't image that finding and taking care of a black cat on the street would ever meant to him to leave his world of simple and entering one of difficult choises and decisions. The cat is revealed to have a human form and be a hikikomori vampire. Now that Mahiru called him Kuro and gave him a collar, the two of them are bonded with more than a coincidence.
The world od Servamps and Eve is opened to him. And together with it, a way for Mahiru to find out more truths, even about himself.