White Mischief (movie, 1987)

A millionaire past his prime and his young wife arrive in Kenya circa 1940 to find that the other affluent British expatriates are living large as the homefront gears up for war. They are busy swapping partners, doing drugs, and attending lavish parties and horse races. She begins a torrid affair with one of the bon vivants, and her husband finds out and confronts them. The husband and wife decide to break up peacefully, but the bon vivant is murdered and all the evidence points to the husband.