Alexandre Vincent Perez feels disillusioned with his fiancée, Laure Marine Delterme, whose idea of romance is a pair of slippers for Valentine's Day. Their once passionate relationship has settled into monotony. To clear his mind, Alexandre visits the beach cottage of his friends Ti and Maude Micheline Presle, where he meets Fanfan Sophie Marceau, Maude's granddaughter, a student studying to become a perfumer. Though they sleep in the same bed that night, Alexandre avoids any physical contact.
As days pass, Alexandre and Fanfan grow closer. Fanfan takes him to an abandoned house where they share intimate conversations. Back at his apartment, Laure tries to reignite their relationship by dressing as an Italian call girl, but the attempt falls flat. Laure's father's gloomy talk of Alexandre joining the funeral business and the bleak future of marriage discourages him further.
Fanfan confesses to a friend that she's in love with Alexandre, but for him, things are more complex. He confides in Ti that all his relationships lose their magic once sex is involved. He proposes a solution: platonic love. To preserve the excitement of unfulfilled desire, Alexandre vows to court Fanfan forever without revealing his feelings or engaging physically. He decides to stay with Laure physically while remaining emotionally connected to Fanfan.
Alexandre and Fanfan share a romantic evening, but when the owner of the apartment they’re using returns, they narrowly escape. Alexandre then creates a magical experience for Fanfan at a television studio, dancing with her in an 1813 Vienna set. However, when he toasts to their "friendship," Fanfan is disappointed and leaves to meet her friend Paul, a sculptor doing a nude study of her.
At home, Laure suspects Alexandre of seeing someone else. He admits it, but insists nothing will come of it. Laure begins planning their wedding, but Alexandre envisions them turning into her parents. Meanwhile, Fanfan, frustrated with Alexandre’s behavior, confronts him. She gives him an ultimatum: choose between her and Laure. Though Alexandre claims to want only friendship, Fanfan knows he is attracted to her.
One evening, Alexandre secretly adds sleeping powder to Fanfan's drink, and while she sleeps, he sensually applies sunscreen on her back. Just as he is about to kiss her, Laure arrives, announcing she is pregnant and the wedding plans are set. Disgusted, Fanfan leaves.
Later, Alexandre discovers that Laure lied about being pregnant, and they break up. He spots Fanfan boarding a bus to Italy but doesn’t chase after her. Ti warns him that Fanfan won’t wait forever. Determined, Alexandre rents the apartment next to hers and installs a two-way mirror to secretly watch her.
Back from Italy, Fanfan surprises Alexandre by bringing Paul along on a weekend trip, making him jealous. Alexandre admits that Laure's pregnancy was a lie, but Fanfan says it’s too late. She pretends to be marrying Paul to provoke Alexandre. He discovers the truth through the two-way mirror—Fanfan is still in love with him.
Fanfan confronts Alexandre, and he confesses that he didn’t chase the bus because he knew their relationship would eventually lose its spark. Fanfan gives him one last chance, telling him to come to her room by 10:00 pm or lose her forever. When he doesn't show up, she moves out, leaving a note that she’s going to see her sister. Alexandre is devastated when he learns from Maude that Fanfan’s sister is dead, implying Fanfan may be contemplating suicide. Just then, the mirror between them cracks, and Fanfan breaks through the barrier. They kiss, and Fanfan gives Alexandre until morning to win her back.
