Set in April 1915, the story begins with 15-year-old Mary Russell, an orphan living with her aunt in Sussex, England. While walking the Sussex Downs, she encounters the retired Sherlock Holmes, now a beekeeper. Recognizing her sharp intellect, Holmes takes Mary under his wing as his apprentice. Their partnership quickly evolves as they collaborate on a case involving espionage, marking the resurgence of Holmes's detective work and the beginning of Mary's training .