On the banks of the picturesque Belon River, in an area where world-famous oysters are grown, a wayward aging actress discovers the corpse of a man. Only a little later, Commissioner Dupin receives a message from the legendary hills of the Monts d'Arrée, a land of legends about fairies and the devil. A dead person, whose identity the police do not know, was also found there. When the commissioner discovers that clues lead to the mafia, Celtic brotherhoods and mysterious druidic cults, he begins to suspect that this will be his most bizarre case yet...