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