Usually, the grown-ups in the Secret Seven series play a regulatory role, and, at times, even forbid the children from getting involved in mysteries. However, it is Peter's and Janet's father who drops this mystery in their lap and suggests that the Seven tackle it. A girl—Elizabeth Sonning—is missing after she is accused of stealing money. Since her grandmother lives in neighbouring Belling Village, Peter and Janet take on the case. Can the Secret Seven follow the clues and catch up with the girl, or will Susie, Jack's troublesome little sister, get in the way?