To negotiate a divorce from her husband, Lord Edgware, the talented American actress, Jane Wilkinson, enlists the help of the famed Belgian detective, Hercule Poirot, and his loyal companion, Captain Arthur Hastings. However, when Poirot visits the aristocrat's mansion, much to his surprise, he finds out that Edgware has already consented to his wife's request; furthermore, he has already sent her a letter. Then, the following day, Edgware is found in his library stabbed to death, and both his secretary and his butler claim that they know the murderer. Who, and above all, for what reason would want Lord Edgware dead?