This 1975 French import leisurely develops the unfolding relationship between Ludovic (Victor Lanoux) who is married to a promiscuous child-woman (Marie-France Pisier) and Marthe (Marie-Christine Barrault) whose husband (Guy Marchand) is an excessive philanderer. Ludovic and Marthe find in each other those qualities of mutuality, affection, and relaxation not available from their spouses.

Writer-director Jean-Charles Tacchella is not content just to probe this affair. He sets it in the context of repeated family gatherings. Relatives have a difficult time responding to the couple's blatant disregard for societal conventions. The moderate interest Cousin, Cousine holds can be attributed to Tacchella's felicitous delineation of Ludovic and Marthe's romance and the clever way he depicts other's reactions to it.