A young woman named Magdalena retrieves a postcard that had been cast into the wind by her biological mother (Bulle Ogier) from a seaside town in Portugal and discovers that she has a twin sister named Maria. From this seemingly introspective opening premise on identity, connection, and history, Deux diverges into unexpectedly abstract, non-intersecting trajectories that involve a schoolgirl attraction with a fellow classmate, a mother's wartime romance, a serial killer who leaves a tell-tale rose on the bodies of his victims, a lonely woman who adopts a fox as a household pet.