There is almost nothing extraordinary that Duff hasn't seen - flying saucers, stray elephants, international spies. So, when he reports finding a real live monkey in the family barn, no one believes him except for his best friend Simon. Simon helps him look after the mysterious monkey, but their friends and family are convinced that their monkey stories are all from their over-active imaginations. Just when everyone has had enough of Duff's tall tales a circus comes to town minus it's star monkey. Could Duff be telling the truth after all?