Two painting are stolen ,in an art gallery and in a painter's house.The burglars always leave the picture of a royal seal after their theft.The local captain in charge of the investigation is not very smart ,but fortunately private detective Roland Gautier takes the affair in hand : it must be a gang .He suspects the shady owner of a nightclub .Gautier is kidnapped and held prisoner on a barge.To get rid of him, the criminals want to drown him.The boss's mistress,Irmène ,remembers that the young man saved her little brother's life and decides to help him.