Damage

• a novel (1991) by Josephine Hart[?] (ISBN 0449911888) about a British politician who, in the prime of life, causes his own downfall through an inappropriate relationship; and

• a film (1992) by Louis Malle[?] based on Hart's book, starring Jeremy Irons and Juliette Binoche, with a screenplay by David Hare[?].

Warning: Wikipedia contains spoilers

The first person narrator of the novel is Dr Stephen Fleming (Irons in the movie), a medical doctor turned politician whose promotion from MP to cabinet member is imminent. Just then Fleming is casually introduced to his grown-up son's enigmatic girlfriend Anna (Binoche) and helplessly falls for her. For as long as it lasts, Martyn, his son, has no idea that his father is having an extra-marital affair with his girlfriend (and later fiancée), and Anna does not seem to mind being a young man's partner and simultaneously his father's lover and object of desire. Fleming enjoys a brief period of sexual bliss meeting Anna in various European cities and having sex with her in unlikely places. Eventually, he buys her a small flat in central London where they meet on a regular basis.

One day Martyn happens to get hold of that address and, curious, goes there to investigate. He climbs up a flight of stairs to the top floor, opens the unlocked door to the apartment, and is shocked to see his father making love to his fiancée. Dazed and utterly confused, he tumbles backwards, hits the low banister and falls down the stairwell. When the lovers have put on their clothes again and hurriedly leave their love nest, Martyn is already dead, sprawled out on the ground floor. In the final scene, we see Fleming, stripped of his political office and living abroad as a recluse, sitting in his solitary room staring at Anna's oversized photograph on the wall.