Barney, Roger, Diane and Snubby love solving mysteries, with the help of Loony the spaniel and Miranda the monkey.Three weeks' holiday near beautiful Merlin's Cove in Wales sounds like great fun to the four friends, and they plan days full of swimming, ice creams and picnics on the beach. But then Snubby gets hold of a letter written in secret code, and the subsequent events make the friends realise that some sinister dealing are taking place.