Iwaki Tomonori is a typical 15-year-old lad who enjoys the company of older women. He develops a crush on his instructor, but he soon finds that she has gotten engaged. He returns home, heartbroken, to discover a 12-year-old girl called Rizel at his door, whom he learns the Japanese government has married him against his choice. Furthermore, he discovers Rizel is no ordinary girl; she is a biochemically created human, and her makers (her "papa"s, as they are sometimes referred to) pledge to waive several expenses of Iwaki's parents in return for the right to reside at their home, and thus Iwaki's life with Rizel starts.