Mizuhara Ryo is a normal high school student, living alone with his younger sister Akane. They were very happy together, at least until a mysterious girl in jet-powered parachute crashes through their window and into their lives. A very beautiful Masuda Shizuka introduces herself as Ryo's new fiancé - him having been chosen as her perfect partner by a secret supercomputer and they are now engaged under a secret national project to address the declining birthrate. And to emphasize the importance of the project, thirty black suited men break into the house, to hold Ryo down and quell his screaming complaints. Shizuka informs Ryo and Akane that she is there to stay!