Mahiru Shirota is convinced that simplicity is superior and that troublesome matters must be avoided at all costs. He considers it troublesome to do nothing and regret it later, and this mindset has led the 15-year-old to take in a stray cat on his way home from school. As he affectionately names the feline Kuro, he has no idea that this chance encounter will trigger an extraordinary change in his daily life. One day, Mahiru returns home to discover something quite odd: a mysterious young man he has never seen before. His ensuing panic causes the uninvited guest to be exposed to sunlight and—much to Mahiru's shock—transform into Kuro! After revealing himself as a mere lazy shut-in vampire, Kuro promises to leave once night falls. However, one disaster after another leads to Mahiru accidentally forming a contract with his new freeloader, dragging him into a life-threatening battle of supernatural servants and bloodthirsty beings that is anything but simple.