Setting off on holiday aboard a borrowed cabin cruiser, The Mary Ann Shaughnessy, the brothers Jonathan and Malcolm Crawford and their friend Joe argue in vain against taking the family's battle-scarred and troublesome bull terrier with them. Bill, due in court soon over an unfortunate '-misunderstanding' with the postman, is sure to cause them problems.

But the boys are very glad to have Bill on board when they sail right into an adventure - one in which their very lives are to depend on the plucky dog's courage and spirit.