When the eccentric Blanche Porter reports that her beloved Mr Cheeters has vanished, it isn't your standard missing persons case. As Nancy Drew soon discovers, Mr Cheeters is a pet chimp. Or is he? Based on a preliminary investigation and information obtained from Blanche's brother, Lawrence, the River Heights police dismiss the case as bogus - doubting that there ever was a Mr Cheeters to begin with. But when Nancy Drew discovers there's a missing diamond necklace as well - she's on the case! Can Nancy, along with Bess and George, recover the great ape and the necklace, or has Blanche Porter made a monkey out of Nancy Drew?