1930s England; spirited teenager Mabel West is determined to live her life on her own terms. Her father agrees to let her leave her stuffy boarding school and seek employment in London. In the city, she works as a junior assistant to the famous Belgian detective, Hercule Poirot. Also accompanied by Poirot's assistant Hastings, secretary Miss Lemon, and Inspector Sharpe, the formidable team solve numerous exciting mysteries. However, Mabel does not forget her family and often travels to the countryside to stay with her great-aunt Marple. This shrewd older woman also has a reputation for solving puzzling mysteries.