At the end of the 18th century, Marianne, a talented painter, is instructing an art class in France when one of her female students inquires about a piece she refers to as Portrait de la jeune fille en feu.
Years earlier, Marianne finds herself on a remote island in Brittany, having been commissioned to create a portrait of a young woman from the gentry named Héloïse. This young lady is destined to marry a nobleman from Milan, but her mother, the Countess, informs Marianne that Héloïse has previously refused to sit for portraits, as she is unwilling to go through with the marriage. Having lived in a convent, Héloïse returned only after her sister's tragic suicide forced her back into societal expectations. To keep her true intent a secret, Marianne poses as Héloïse's hired companion, allowing her to accompany her on walks along the breathtaking coastline while memorizing every detail of her features.
Once Marianne completes the portrait, she struggles with the urge to betray Héloïse's trust and confesses her true purpose for being there. After Héloïse voices her criticism of the painting, feeling it fails to capture her essence, Marianne decides to destroy the piece. After witnessing the destruction, Marianne explains to the Countess that she can create a better representation of Héloïse. To Marianne's surprise, Héloïse declares that she will pose for her, shocking the Countess, who then permits Marianne just five days to finish the new painting while she goes to the mainland. Throughout this time, Marianne is haunted by visions of Héloïse in a wedding dress. One night, they delve into the myth of Orpheus and Eurydice, debating the true motive behind Orpheus's fateful glance at his wife, thus bringing her back to the underworld. Later, they attend a bonfire gathering where the women sing, and in a moment of peril, Héloïse's dress nearly goes up in flames.
The following day marks a turning point as Marianne and Héloïse share a passionate kiss, leading to an intimate night together. Over the next few days, their bond deepens, as they support Sophie, the housemaid, in her desperate decision to seek an abortion due to her unexpected pregnancy. As the impending return of the Countess looms over them, Marianne sketches a drawing of Héloïse to keep her memory alive. In an intimate moment, Héloïse requests Marianne to draw a nude sketch of her on page 28 of her book. With the portrait finally finished, the Countess expresses her approval. The next morning, as Marianne prepares to leave, she hears Héloïse’s voice saying, "Turn around." When she does, she sees Héloïse in her wedding dress, a heart-wrenching sight.
In the present, Marianne recounts that she encountered Héloïse two additional times since then. The first occurred at an art exhibition where Héloïse was depicted in a portrait, holding a book with a child by her side, subtly revealing page 28. The second encounter took place at a concert in Milan, where Marianne glimpsed Héloïse in the balcony among the patrons. Unseen, Marianne witnesses Héloïse with tears in her eyes, both smiling and crying as she listens to the orchestra performing the Presto from Vivaldi’s Four Seasons, the very piece of music Marianne had played for her on the harpsichord years before.
