Shungo Ninomiya is a mostly Ordinary High School Student... One day, his sister Ryoko sends Mayu and Mikihiro Tsukimura, two siblings, to live with him to help Mayu overcome her problems. She has a fear of men... and she also happens to be a succubus who can't turn off her power to attract them. Shungo, always well-behaved around women, seems to have some resistance, though Ryoko pushes this to the limit by having them do "training" like sharing a bed and making mock confessions. And then there's the student-council chair, Reika Hojo, who somehow ends up becoming their maid...