Eight Men in a Castle.

The Paladines write crime novels together. In a cinema, they happen to see a report on the consequences of a shipwreck in which Lieutenant Dupuis tells about the death of an old man in the lifeboat carrying the survivors. Husband and wife agree that this could be the starting point of a new story and go and see Dupuis to get more details about the drama. When they come to his hotel Dupuis is dead with a bullet in his head. Police characterizes the death as suicide but Paladine has found a train ticket in the dead man's pocket. With his wife he takes the train to the specified destination, somewhere in the provinces...