This self-contained individual, this man who had rarely allowed even his sturdy, ex-Army companion Watson to confront real risk, who had habitually ... held back, been cautious, kept an eye out and otherwise protected me; this man who was a Victorian gentleman down to his boots; this man was now proposing to place not only his life and limb into my untested, inexperienced and above all female hands, but my own life as well. (p. 257)
One day in 1915, a teenage girl with her nose in a book practically trips over a retired detective on the South Downs. It's not an auspicious meeting: he mistakes her for a boy, earning the scornful rejoinder that it's probably a good thing he's retired, then.
Despite their initial antipathy, it quickly becomes clear that this is a meeting of minds. In Mary Russell (fifteen, feminist, brilliant, independent, Jewish, orphaned, and unhappy), Sherlock Holmes finds an equal partner of a kind he's never had: Russell, for her part, finds a father-figure (which sits slightly uncomfortably with me, given developments in later books), a mentor and a friend.
The Beekeeper's Apprentice spans the first four years of their association, and a variety of cases (from local robbery to German spies to a slow-cooked revenge). Mary Russell soon discovers that Holmes's retirement is more of a polite fiction; she gets to know the man behind the stories, which have been somewhat embellished ("I deduce, Miss Russell: Watson transforms") and, in the process, comes to understand herself rather better too.
The characters are true to canon, though I'm not keen on King's Watson, who is as bumbling and slow as any of the early film/TV adaptations that made him a comic foil for Holmes. Mrs Hudson is delightful. (And sharp.) And I can certainly extrapolate this Holmes -- ageing gracefully but in full command of his formidable skills, doing intelligence work for Mycroft, bored to distraction once the war's over and he's left without purpose -- from Arthur Conan Doyle's creation.
If I call this 'fan fiction' it's not in any derogatory sense. It takes canon, shares the author's knowledge, affection and joy therein with the reader -- who may well know more about Holmes than Mary Russell does, at least initially -- and uses that canon as the foundation of something new. I confess when I first heard of Mary Russell, I did pointedly enquire if her middle name was by any chance 'Sue': but Russell is a well-rounded individual in her own right, and though she is an excellent counterpart to Holmes, it's Russell herself rather than her mentor (or her relationship with him) who is the focus of this and subsequent novels.