Mahiru Shirota is strongly of the view that straightforward issues are safest and that complicated stuff can be stopped at all times. It's confusing doing nothing and regretting it later — so this theory has driven the 15-year-old to pick up a homeless cat from school on his ride home. As the feline Kuro is affectionately called, little does he realize this unlikely encounter would spark an exceptional shift in his everyday life.
One day, Mahiru comes home to see something very odd: a strange young man he'd never seen before. His resulting fear culminated in the exposing of the uninvited visitor to sunshine and – much to the surprise of Mahiru – turning him into Kuro! Kuro plans to depart as night falls, after he exposes himself as a pure lazy shut-in vampire. Nevertheless, one tragedy after another causes Mahiru to unintentionally establish a contract with his latest freeloader, pulling him into a life-threatening war of supernatural servants and bloodthirsty creatures which is anything but plain