Dead Man's Folly (1986) on Netflix

During a murder hunt game at a country house, to which Hercule Poirot is invited as an "expert", a real murder occurs.

Hercule Poirot (Sir Peter Ustinov) is called in by his crime writer friend, Ariadne Oliver (Jean Stapleton) and discovers that she is troubled. She has been asked to create a "Murder Hunt" game for a fair at Nass House and she is puzzled with all the help she is getting. Poirot and his assistant, Captain Arthur Hastings (Jonathan Cecil) arrive at the fair to see what is going on. They find a couple on the brink of divorce, a rich Lord and a dizzy Lady, an old lady, trapped in the horrors of the past and a womanizing architect. Things take a turn for the worse when during the "Murder Hunt", the girl playing the "dead" body is murdered for real, an old man's body is pulled from the local lake, and the Lady of the manor goes missing when a face from her past shows up. It is clear to Poirot that someone is playing the game for real, and he sets out to discover who it is.

—Lee Horton <Leeh@tcp.co.uk>