Dr. Stephen Fleming, portrayed by Jeremy Irons, is a British cabinet minister who enjoys a seemingly idyllic life with his loving wife, Ingrid (played by Miranda Richardson), and their young daughter, Sally (brought to life by Gemma Clarke). Stephen's older son, Martyn (Rupert Graves), is an up-and-coming journalist.
During a lively party, Stephen encounters a captivating half-French woman named Anna Barton (Juliette Binoche). She announces herself as a close friend of Martyn, but it quickly becomes evident that there is a strong attraction between Stephen and Anna. The tension escalates when Martyn visits his parents in London with Anna in tow, revealing their romantic involvement. Despite Anna's relationship with his son, she arranges a secret meeting with Stephen in her apartment.
As Stephen's fascination with Anna intensifies, he irrationally decides to meet her in Paris after an international summit, opting for her company over returning home. In a bold display of passion, Stephen and Anna become intimate in the daylight while Martyn is resting in a nearby hotel room. Stephen's obsession grows to the point where he compels himself to spy on Anna and Martyn, noting their time together from across the street. He yearns to be with Anna despite the potential fallout with his son.
Anna, however, reassures Stephen that their affair can continue as long as she remains with Martyn. Yet, complications arise when Stephen encounters a man named Peter Wetzler (Peter Stormare) in Anna's apartment. Peter, unaware of Stephen's involvement with Anna, reveals that he and Anna were once lovers. Overwhelmed with jealousy, Stephen initially confronts Anna, who explains the traumatic circumstances surrounding her brother’s suicide. Her elaborate recounting of grief and loss brings them back together physically.
The dynamic shifts when the Fleming family gathers at their countryside estate for Ingrid's birthday celebration. During the festivities, Martyn's surprise engagement announcement to Anna rattles Stephen, intensifying the emotional turmoil. Later that evening, after a heated encounter with Anna, Stephen is caught in a compromising position by Sally, who has questions about what she witnessed. Stephen lies, claiming he was simply comforting Anna.
As tensions mount, Anna's mother becomes suspicious of the connection between Stephen and her daughter and advises him to end the affair. Although Stephen initially attempts to do so, he quickly backtracks after being tempted by Anna, who sends him keys to a flat where they can meet discreetly. However, their secret rendezvous takes a tragic turn when Martyn inadvertently discovers them in bed together. The confrontation results in a devastating accident where Martyn falls to his death, leaving Stephen in shock and Anna in silent despair.
Following the tragic events, Stephen's infidelity is exposed, unleashing a media storm that surrounds him. Inside his home, an anguished Ingrid questions Stephen's love for her in light of his betrayal, leading to the implication that their marriage may be ending. Subsequently, he is forced to resign from his cabinet position and withdraws from the public eye, retreating to an undisclosed place in Europe.
In a reflective narration, Stephen reveals that Anna has since had a child with Peter. He continues to carry the weight of his choices, captured in a life-sized photograph of himself with Anna and Martyn hanging in his flat, a haunting reminder of love lost and lives irrevocably changed.
