In 1915 Sherlock Holmes is retired and living quietly as he undertakes a study of honeybees in Sussex. Fifteen-year-old Mary Russell literally stumbles upon him as he is working with the bees. As Mary gets to know Holmes, she displays an intellect that impresses even the Victorian detective---no easy feat. Soon they are asked to investigate the kidnapping of the daughter of an American senator, a crime of national significance that proves to have clues that dip deeply into Holmes' past.