Sandra Kienzler, portrayed by Léa Seydoux, is a young widow raising her daughter, Linn, while working as a translator in Paris. Amidst her responsibilities, she also looks after her father, Georg, a former philosophy teacher suffering from Benson's syndrome—a neurodegenerative disease that progressively impairs his cognitive functions and vision.
One day in a park, Sandra encounters Clément, an old friend of her late husband, played by Melvil Poupaud. Clément, a married chemical cosmologist who has just returned from an expedition to the North Pole, sparks a connection with Sandra, and they agree to dine together. During a visit to his lab, a moment of intimacy transpires—a kiss that leads them into a discreet affair.
Meanwhile, Sandra's mother, Françoise, who has divorced Georg and remarried, remains a constant support in caring for him. She expresses her deep concerns about Georg's deteriorating condition, revealing that he can no longer manage daily activities and suffers from hallucinations. After much deliberation, they decide that the time has come to place him in a nursing home where he can receive comprehensive care. However, the family faces challenges—public home placements can take years while private options are out of their financial reach. In the interim, Georg is admitted to a hospital. There, Sandra meets one of Georg's former students, who fondly remembers him and wishes to write to him. Sandra offers her email as a point of contact.
As Sandra and her family begin to clear out Georg's apartment, filled to the brim with books, she rescues a few volumes for herself and intends to give the remainder to the former student. At the hospital, Georg frequently mentions his long-time girlfriend, Leïla, raising concerns about his awareness of his surroundings. As his condition worsens, he ends up temporarily in a public home, where he experiences further confusion and isolation. With the staff's notice for a prompt transition to a new facility, the family resolves to move him to a private home.
Meanwhile, Sandra's romance with Clément begins to show strain. Clément struggles with leaving his wife and son, while Sandra grapples with the reality of being merely his mistress. Caught in a park with Sandra and Linn, Clément spots his wife's friends and decides to hastily retreat. After a heartfelt discussion at home, he assures Sandra that he will return once he makes a decision about his marriage.
While sorting through her possessions, Sandra discovers a small notebook Georg had used to document his health and a rough draft of an autobiography that he had once dreamed of completing. At their Christmas dinner, she shares this discovery with her mother and sister, revealing a notebook titled "An einem schönen Morgen" (translated as "One Fine Morning").
As summer unfolds, Clément finally frees himself from his marriage and returns to Sandra. The trio—Sandra, Linn, and Clément—visit Georg in his new private residence and talk about a recent trip to Italy. Confusion lingers as Georg continually asks for Leïla, unaware of his surroundings and unable to recognize his own family. In a touching moment, they join other residents for a group activity, singing "Mon amant de Saint-Jean" together. Overwhelmed with emotion, Sandra leaves with Clément and Linn.
The story culminates in a poignant moment as Sandra, Clément, and Linn ascend the steps of the Sacré-Cœur Basilica. Upon reaching the dome, they gaze out over the beautiful city of Paris, reflecting on their intertwined lives and the bittersweet nature of their experiences.
