4David Blake and his sister Sally have grown up with the legend of the Blake treasure, buried on one of the islands off Saturday Cove, a fishing village on the coast of Maine. One day, while lobstering in the bay, David and Sally seek shelter from a summer storm in an abandoned house on Blake's Island. There they find a musty chart that seems to be a definite clue. Curious and excited, they decide to solve the family mystery.

But Sally loses the chart, and David reckons without the troublesome interference of Roddy McNeil, a newcomer to Saturday Cove. Roddy's father threatens to buy Blake's Island and the children fear the treasure will be lost to them forever.

The mystery is heightened with the discovery of an unidentified eavesdroppers, the disappearance of a rare pewter button, and the fishermen's growing suspicion that it is David who is stealing lobsters from their traps.

Barbee Oliver Carleton successfully captures the special flavor of the Maine coast, and brings to children a story of unusual imagination and excitement.