When her divorced mother remarries, high schooler Mitsuki Kanzaki gains a stepbrother, Yuuya, and then, once their parents leave the country, a housemate she has never lived alone with before. Into that arrangement drops Hiyori, the ghost of a girl who died before she could confess her feelings for Yuuya and who now possesses Mitsuki, along with a chastity belt called the TST that fills as Mitsuki's own feelings toward him turn romantic. This light novel is an adaptation of the original manga, which also produced a TV anime.