Nathalie is in fact jealous of everybody. Her daughter, who has a doting boyfriend, her best friend, who seems to have a perfect marriage, and a new young female teacher who has arrived at the college where she teaches. The jealously becomes all-consuming and leads to alienation of all around her, including a new love interest, who she throws out after accusing him of ogling her daughter.
Her daughter is a ballet dancer on track for an audition at the Opéra de Paris. Or at least she was until Nathalie almost kills her by feeding her an allergen that brings on anaphylactic shock. Her daughter moves to her father's and Nathalie's life is descending into abject depression.
She has unsuccessful counselling, having been told by her general practitioner that he couldn't help her. She takes up Yoga, where she invariably falls asleep on the mat, and swimming, where she meets an elderly woman who seems to put her back on track to some form of recovery. When this woman dies after a fall, Nathalie asks the date she threw out to accompany her to the funeral, which he does very reluctantly. This is the real turning point.
Things sort of work out in the end but not quite happily for everybody. In the final scene, Nathalie visits a couple, neighbours who had invited her for an aperitif on many occasions. But the young man was alone, his girlfriend having left because of his jealousy. He asks Nathalie if she's jealous, to which she replies "Moi, non !"