Reunited with Tinker, who featured in an earlier adventure: the Famous Five are invited to camp out in a field close to his house (in which Professor Hayling, Tinkers father who is a scientist lives). When a Circus arrives, and sets up camp, close by the Famous Five are contented with the mild excitement of watching the Circus folks antics, and getting to know them, but then real commotion strikes, when a tower in the grounds of Professor Haylings house is robbed. Can the Five track down the culprit? And what has the mystery got to do with Kirrin Island?