Roger, Diana and Snubby are looking forward to spending a good holiday at Rockingdown Cottage. Then they learn they are to have a tutor for extra coaching. But is the tutor, Mr. King, all he appears? The children think he's a fraud.

Then their new friend, Barney and his pet monkey disappear. He's trapped beneath Rockingdown House. How can he get a message to the others before it's too late? For Barney has tumbled on a startling secret...