Easy to kill

Luke Fitzwilliam, a retired colonial policeman has returned to England and chances to converse on a train with a woman who reminds him of a favorite aunt. She informs him that she is reporting three murders to Scotland Yard and is hoping to prevent a fourth, that of a village doctor. Before she can do so, she is killed by a car, and a short time later the doctor she mentioned is killed. Fitzwilliam decides to investigate these five deaths.