It’s December 1875. Mary Balfour is a rich, precocious 13-year-old girl. Her parents are mysteriously absent and Mary, in spite of her youth, presides over a house and servants in the Hampstead section of London. She keeps a notebook about unsolved crimes and follows the crime news religiously. She deduces that some private person has been helping out the official police, and, with the help of her governess Hannah, she finds out that the private person is Sherlock Holmes, who has just started his consulting detective business from his lodgings on Baker Street. Mary decides to become Holmes’ apprentice. She meets with Holmes and his friend Dr. Watson and demonstrates her deductive abilities, but Holmes turns her down, citing her youth as the reason. Mary, hurt and angry with Holmes for underestimating her, decides to show him up by committing a crime not even he can solve. She enlists the help of a young criminal, about her age, named Arthur Best, who teaches her to pick pockets and locks, to burgle houses, and to get about London by following back streets and rooftops. For her lessons, she slips out of her window each night and meets Arthur on the snow-covered Hampstead Heath. After several months of practice under Arthur’s tutelage, Mary discloses her plan to Arthur. But he's not sure it will work. Mary, with the help of Arthur, her servants, and some disreputable friends, pits her intelligence against Sherlock Holmes to commit a crime involving the highest social circles of England. Will she succeed?