A Monstrous Regiment of Women isn't my favorite of the Holmes/Russell novels, but that's a little like saying dark isn't my favorite type of chocolate. It's still chocolate, and therefore by definition far better than many another thing.
Mary Russell has graduated from Oxford, is about to turn twenty-one and achieve separation from her horrible aunt … and her joy at these two events is dampened a bit by the peculiarities of her evolving relationship with her mentor Holmes, never an easy person to deal with at the best of times. It is while Mary is, literally, on the run from him that a friend from school runs into her by chance, and she needs help. Not financial, by any means, but in most other ways: her fiancé has come home from the war in something less than the shape she saw him off in, and there's also something going on at a church she has begun to frequent… Mary, the logical-minded theology scholar, raises eyebrows at the "church", but she agrees to help, if for no other reason than that her partnership with Holmes is being challenged by an extraordinary circumstance and she needs occupation. Preferably some occupation in which she can prove herself to be independent of her iconic mentor. She is drawn into the orbit of Margery Childe, the proto-feminist mystic head of the New Temple of God, and her expectations are upended. The obvious chicanery she anticipated is nowhere to be seen, and instead Childe turns out to be a small woman of tremendous charisma – and, perhaps, something else.
Russell is on her own through much of MRoW. Holmes swoops in to bear off the shell-shocked and drug-addicted fiancé to an Edwardian rehab facility, and appears here and there for the rest of the story, but this is mostly Russell's investigation. She grows and expands in this book, not all in positive ways as the story takes a wild left turn into kidnapping and drug addiction. It's painful, and difficult – and melodramatic and improbable – and Laurie R. King sells it.