Iwaki Tomonori is an ordinary 15-year-old boy with a preference for older women. He develops a crush on his teacher, only to discover shortly after that she has become engaged. Heartbroken, he returns home, where he finds a 12-year-old girl named Rizel. He learns that the Japanese government has forced him into marriage with her against his will. Moreover, he discovers that Rizel is not an ordinary girl; she is a biochemically engineered human. Her creators, often referred to as her "papa"s, promise to waive several of Iwaki's parents' bills in exchange for permission to let her stay in their house, and thus Iwaki's life with Rizel begins.