The story begins in 1915 when a young, brilliant Mary Russell stumbles upon the retired Holmes in Sussex. Holmes is impressed by her intellect and deductive skills, and he takes her under his wing.

As they train together, Mary and Holmes form a strong bond and face various cases, ranging from solving local mysteries to thwarting an international conspiracy. They encounter dangerous adversaries, including a master criminal who targets them personally. Throughout their adventures, Mary proves herself to be a formidable partner to Holmes.