In an early 20th century where magic, to some degree, works, but not much else is changed, Dorothy Boone is the very frustrating daughter of a very respectable family. In time, the mutual frustration becomes so great that she moves in with her grandmother, Lady Eula Boone. She gets a very unexpected education, and eventually inherits her grandmother's maps, books, artifacts, and career. I'm not clear on exactly why she is thereafter Lady Boone, but this isn't exactly our world, and I choose not to worry about it.
Miss Trafalgar starts out as Tall Girl, a child in a desperately poor Ethiopian village. She and others get traded off to some men who claim they are looking for girls to train as nurses, etc. They have a different plan for the girls, of course, or at least whichever one of them seems most suitable.
They need a body for a supernatural creature from the past to inhabit, so it can rule the world. It doesn't quite work out the way they expect.
Miss Trafalgar and Lady Boone both become notable archaeological adventurers, and rivals in the field. Their mutual competitiveness and dislike is strong.
And then one day an obscure group arranges events that will either kill them both, or set them at each other's throats. They do the same for nearly all the rest of the profession, or at least attempt to, and the death toll is terrible.
Lady Boone and Miss Trafalgar are left with no choice. As distasteful as it is, they will have to work together, to find their common enemy and prevent disaster for England and the entire world. Because Trafalgar's old enemies went underground, but they didn't go out of business. They have a much, much more dangerous plan than simply taking over the archaeological profession. Boone, Trafalgar, their friends, and the reader have a lively adventure in post-Great War Europe, with airships and magic and nefarious villains everywhere. It's a lot of fun.