"Do the Secret Seven want to make themselves really useful?" said Peter and Janet's father, to them both, after reading the newspaper. He tells them about a young girl that has gone missing after stealing some money from her teacher's desk. Her parents are abroad and her brother is in France, according to what he'd just read. But the girl, whose name is Elizabeth, has been spotted in the next village of Belling. The Secret Seven think this will be a nice little mystery to clear up, if they can find this missing girl.