A man in a happy marriage with two kids begins an affair, sincerely feeling he has enough love for both women and that neither one will be loved less. To start with, it's absolutely beautiful to look at. Varda always seems to know exactly what to do with the image, where to put the camera, which direction to move, when to cut, what color to fade to; everything is absolutely perfect.
Moreover, the film is completely fascinating first because Varda deals with her subject with a rare honesty and forgiveness. Not a single character is unlikeable. Even if you see error in the husband's thinking, it is clear he believes with all of his heart that he truly can love both of these women at once and you sympathize with his sincerity. The wife is easy to care for, a good mother and very devoted, and the mistress is not someone you feel compelled to hate, either. She's not out to break up this marriage and she seems to really need this love.
And what makes the film endlessly interesting is in how ambiguous Varda is about her own feelings. She never leads you to pick a side, never encourages you to see one specific viewpoint or leave the film feeling a particular way about what happened. While the music (Mozart is used throughout most of the film) in the last 15 minutes would seem to suggest anger at the way things have turned out, you can also look at the early stages of the film and see the image of the idyllic family with pastoral music as too perfect a presentation, one that is not entirely believable. Varda even hints at this herself; after we've watched about five minutes of this family picnicking in the woods, she cuts almost immediately to nearly the same image in a TV advertisement, suggesting that a marriage that happy only exists in commercials to begin with.