Ozaki Haruta is just an ordinary son of a locksmith. One day, he’s called to help a strange woman in a maid outfit, Marui Rumine, with unlocking a door. He succeeds, and on the other side are weird horrors that Marui seems to have the power to destroy. Is she a maid? Is she a witch? Before Ozaki knows it, he has a housekeeper with an extraordinary lock right at her crotch where her powers are sealed!