Years after their reconciliation, Antoine Jean-Pierre Léaud and his wife Christine Claude Jade are in the midst of divorce proceedings, each taking turns caring for their son Alphonse Julien Dubois while the relationship strains under a web of past choices and present evasions. Antoine is seeing Sabine Dorothée, a quick-witted record shop clerk, as he navigates the uneasy balance between responsibility and longing.
The story pivots around a chance encounter that reveals how fragile memory can be. After the couple exit the courthouse, Antoine is unexpectedly spotted by his ex-girlfriend Colette Marie-France Pisier, a sharp defense lawyer who still carries questions about their shared history. Intrigued, she heads to a bookstore to learn more about the semi-autobiographical novel Antoine once wrote, wondering how much he has really learned from their past. The bookstore’s owner, Xavier Daniel Mesguich, is Colette’s current boyfriend, and her uncertainties about his true feelings add another layer of tension to the unfolding drama.
At the Gare de Lyon, Colette boards a train bound for Aix-en-Provence for a court appearance, while Antoine is at the same station to see off Alphonse, who will join a summer music camp on a train across the platform. A tense moment occurs when Antoine phones Sabine to skip his role at a party, claiming a need to attend to Alphonse’s trip. Sabine, sensing secrecy in his behavior, asks questions about meeting Alphonse, and their conversation ends with a clipped, unresolved goodbye.
In a bold turn, Antoine impulsively jumps onto Colette’s train as it departs. Onboard, Colette reads Antoine’s book and discovers how he has reshaped events from their life together to present himself in a more flattering light. The two share a charged moment in the dining car, and she invites him to her sleeping car. As they talk about a new novel he’s supposedly writing, Antoine recounts the curious way he first met Sabine: a torn photograph destroyed in a phone booth by a man who was wildly in love with her, a memory Antoine reconstructs with tape and insistent fascination.
This revelation deepens Colette’s disappointment in him, exposing how little he has actually changed. When the train begins its relentless motion, Antoine attempts a kiss, but Colette pulls away, insisting that he hasn’t learned from their past. A dramatic slip occurs as Antoine helps Sabine during the journey—while attempting to hide, he accidentally drops the photo. Realizing the truth of his tale, Colette decides to safeguard the photograph, recognizing its significance to Sabine and to Antoine’s confession.
Back on solid footing with Sabine, Antoine tries to make amends for a hurried goodbye and a careless gesture, but Sabine’s response—returning the letters he wrote to her—signals a more honest reckoning than he anticipated. He understands that to rescue his relationship, he must own up to how he learned about her, yet the missing photograph still haunts him. As this tension builds, Lucien Julien Bertheau, a former lover of Antoine’s mother, re-enters the orbit, guiding him toward a place of memory and truth that he had never sought before.
Meanwhile, Colette examines a clue hidden on the back of the photograph and learns that Sabine shares a last name with Xavier. What she assumes—at first—that Sabine could be Xavier’s secret wife—shifts when she discovers, upon returning to Paris, that Sabine is in fact Xavier’s sister. This revelation adds a new twist to the relationships entangling the group and prompts Colette to return to Sabine’s apartment to hand back the photograph—only to find the building empty and Christine on the stairs instead. The two women speak honestly for the first time, discussing Antoine, their lives, and the children who have shaped their decisions.
Christine reveals the cause of her divorce: she caught Antoine with a close friend, Liliane Dani, during a summer holiday. The admission is balanced by Colette’s own confession—that she left her husband after their daughter died in an accident. The conversations on those stairs illuminate two women who have learned to protect their own lives even as they navigate the men who once bound them together.
With new resolve, Colette decides to tell Xavier how she feels and chooses to reunite with him at the bookstore, where a kiss finally seals their renewed connection. Christine, meanwhile, mails Sabine’s photograph to Antoine, a gesture that forces him to confront the truth about how he and Sabine met. When he shows the photo to Sabine and recounts the real origin of their romance, Sabine is moved and their bond softens into reconciliation.
In the end, the film traces a delicate arc of memory, accountability, and forgiveness. The story moves from courtroom tension to train-car confession, from misread motives to honest conversations, and from the ache of past mistakes to the fragile promise of a future where love is tempered by responsibility. The momentum of the relationships—between Antoine, Christine, Colette, Xavier, Sabine, and the people who orbit them—creates a measured, reflective portrait of romance: imperfect, evolving, and ultimately human.
