Damage

Dr. Stephen Fleming (Jeremy Irons), a British cabinet minister, lives a pleasant life with his wife Ingrid (Miranda Richardson) and young daughter Sally (Gemma Clarke). His older son, Martyn (Rupert Graves), is a rising journalist. At a party, Stephen meets a young half-French woman named Anna Barton (Juliette Binoche), who introduces herself as a close friend of Martyn's; it is apparent, however, that Stephen and Anna are instantly attracted to each other.When Martyn visits his parents in London, he brings Anna with him, revealing that they are romantically involved. The sexual tension between Stephen and Anna is clear, though their partners are oblivious of this. Despite her relationship with Martyn, Anna arranges a tryst with Stephen at her apartment.Stephen's obsession with Anna deepens. After an international summit adjourns in Brussels, he travels to Paris to meet her instead of going home. While Martyn is sleeping in a Paris hotel room, Stephen and Anna have sex in an open doorway in broad daylight. Afterwards, Stephen checks into a hotel across the street from Anna's and Martyn's, spying on the couple through his window. Eventually, his infatuation reaches a point where he desires to be with Anna permanently, even at the risk of destroying his relationship with his son. Anna dissuades Stephen from doing this, assuring him that as long as she is with Martyn, he will always have access to her.During a visit to Anna's apartment, Stephen finds that another man named Peter Wetzler (Peter Stormare) is already present. Peter unaware of Stephen's affair with Anna tells him that they are former lovers. A jealous Stephen initially assumes that Anna is cheating on him and, when Peter leaves, confronts her. Anna denies carrying on with Peter, and then describes in detail the death of her brother, who had committed suicide after expressing an incestuous desire for her. She recounts that she had compulsively slept with Peter in order to grieve over her brother's death, which she had witnessed. Stephen is placated by this explanation, and the couple once again have sex.The Flemings travel to their country estate to celebrate Ingrid's birthday. During a meal there, Martyn announces his engagement to Anna, which visibly disturbs Stephen. That night, after Stephen receives oral sex from Anna, Sally catches them outside of Anna's room. Soon after, an anxious Stephen lies to Sally about what she saw, telling her that he was merely trying to console Anna about the upcoming marriage.Later, the Flemings have lunch with Anna's mother, who notices the awkward behavior between Anna and Stephen. She correctly deduces that they are having an affair and, during a car ride with Stephen, warns him to put an end to it.Stephen initially complies and calls Anna to end their relationship. He tries to confess his misdeeds to both Martyn and Ingrid, but ultimately shies away from doing so. However, after Stephen succumbs to temptation and anonymously phones Anna's apartment, she mails him keys to a renovated flat where they can meet. When Anna visits his home, she tells Stephen that she couldn't marry Martyn without being with him.They go to the flat at an appointed time and begin another raucous tryst, but Martyn having been guided to the flat by chance finds them in bed together. Stunned, he falls over a stairway railing and plunges to his death. A devastated Stephen clutches Martyn's body while Anna silently leaves the scene.Stephen's affair is exposed and becomes the subject of a media frenzy, with a mob of reporters convening outside his house. Inside, an anguished Ingrid reacts to the day's events by questioning whether Stephen had ever been in love with her. It is strongly suggested that their marriage end.Stephen, now publicly disgraced, is also forced to resign from his cabinet position. He meets Anna's mother at a hotel and finds that Anna is staying with her; they say nothing as they meet for the last time. Stephen withdraws from public life and retires to an unknown location in Europe. In narration, he reveals that Anna now has a child with Peter. In his flat, a photo of himself with Anna and Martyn, blown up to life size, hangs on his wall.

Question: How did Martyn find out about Anna and Stephen's affair?
Answer: He found them in bed together.