Truffaut juggles an Hitchcockian suspense/thriller with deepening sexual obsession. Louis (Jean-Paul Belmondo) owns a tobacco plantation and cigarette factory on Réunion Island, but it's lonely work — so he sends away for a mail-order bride.
Much to his surprise, the beautiful young Julie Roussel (Catherine Deneuve) arrives by ship (the Mississippi Mermaid of the title), looking nothing like the picture he had received by mail. Louis quickly falls for Julie, while discovering that she is decidedly not the woman with whom he had been corresponding.