A young man brings Inspector Torrence a message he found in a bottle drifting on the Seine. It is a desperate call; an old man is sequestered in an abandoned mill that water is gradually invading ... The O Agency is investigating ... In order to be able to touch the heritage of Chauffier-Mignot; Jean Dassonville wants to marry Cécile the daughter of the family. The father not agreeing, he decides to kidnap him and lock him up in an old water mill. But Chauffier-Mignot refuses to sign the marriage authorization and even succeeds in sending a message in a bottle thrown into the river ...