in reunion island in the indian ocean and the owner of a cigarette factory louis mahé ( jean paul belmondo ) be engage through correspondence with julie roussel and he do not know her . when julie arrive in the island to get married with louis and he wait for her in the dock but louis do not recognize julie in the passenger vessel and find that she be totally different from the picture she have send to louis . they get married and louis share his bank account with her . when julie sister write a letter to louis ask her sister to write to her and louis discovers that the woman be not julie that be miss . far and he find that the woman have clear his bank account and leave the island . louis and julie sister hire an efficient private detective comolli ( michel bouquet ) and louis travel to france seek the woman and but he have a nervous breakdown in nice and be submit to an intense sleeping therapy in a clinic . he recover and find that the woman and actually marion vergano ( catherine deneuve ) and work in the phoenix club privé in antibes and life in the low budget monorail hotel . louis break in her room and when she arrive from the club and she tell that she be happy with him but her former dangerous lover richard have blackmail her . louis be still in love with marion and escape with her to the countryside . but comolli be chase marion in france accuse of murder julie . la sirène du mississipi be a film noir by the great director or writer françois truffaut and with an unconventional love story of passion and murder and love that hurt . the femme fatale catherine deneuve be astonish and probably in the top of her beauty and be delightful to see her face and the topless scene on the road and in the room . jean paul belmondo be very athletic and and the sequence when he escalate the wall of the hotel be impressive . catherine deneuve make this film worth and give credibility to the passion and lust of louis . my vote be eight . title ( brazil ) represent a sereia do mississipi ( the mississippi mermaid ) .