Yearning for a little adventure in life, Anne Beddingfeld gets far more than she bargained for: while at the tube station, she sees a man fall to the tracks and die. A man dressed in brown, claiming to be a doctor, pronounces him dead and hurries away, dropping a scrap of paper with the peculiar words "17-122 Kilmorden Castle". The police conclude the death was an accident, but Anne suspects otherwise. Lord Nasby, the press tycoon, agrees to hire her to investigate the matter. Armed with a small inheritance and the scrap of paper the mysterious man in the brown suit left behind, she sets off on her mission — a mission that takes her from England to South Africa, and that leads to the discovery of missing diamonds, another murder, and attempts on Anne’s own life.