The movie begins on a light note, with neighbour and writer Jeff Marle flying a kite near Draguignan (Var), which lands right on the ass of Julie Wormster (Romy Schneider), who is sunbathing naked on her back lawn, her husband Louis being absent at the time. Jeff timidly opens his mouth and Julie tells him to get it, which he does and then she asks provocatively him if there's anything else he wants. Cut.
Jeff becomes friendly with Julie and Louis, although we can see there's tension between the married couple because Louis is a hopeless whisky drinker, and when he staggers off to bed Jeff and Julie have passionate sex on the carpet. Soon the lovers form a murderous pact: Julie will club her drunken husband to death, Jeff will cart the body off to his boat and dump him in the sea. Julie, of course, will draw Louis's money from the bank.
So the cops question Julie, who is only too ready to provide an alibi for Jeff, who was far away driving the car Louis had 'given permission' for him to drive. She also remarks that her husband may have fallen overboard as he has a bad heart. Trouble is, the cops can't find a drowned body, blood found on the boat belongs to Jeff not Louis, and the car Jeff was supposedly driving went off a cliff. Furthermore, Julie walks right into it by going to the bank and enquires about her husband's money, and he's emptied the account. Furthermore, his doctor reveals that he has no heart problems.
There are of course more twists before we discover that both Louis and Jeff are alive. The dénouement comes thick and fast.