An improbable situation is created in which four children, manning their own boat, explore the river Meuse and its tributaries in France, often docking to sightsee. At the castle of Namur, they hear how the gold of the Spanish tyrant Don John was stolen and never recovered. After some pertinent correspondence, the children investigate, the mystery in detail and eventually decipher a riddle in a book of verse which leads to a solution. In a kayak, they return to the underwater cave in the river Lesse, and there, buried in a stalactite, they discover the treasure after a great deal of effort and maneuvering. A grandiose idea realized by a series of unreasonable situations bordering on absurdity, precludes favorable comment, despite Pilkington reputation.