The Astonishing Adventure of Jane Smith is a fun little one-shot mystery with some of the staples: two girls who are so nearly identical in appearance that one can impersonate the other and briefly fool even her own father, a plucky girl getting into deadly danger for the sake of the realm (even though she has no training and very little knowledge of the situation), a love story (or two) featuring jilted lovers reunited through circumstance, secret underground passages, etc.
Jane is pretty much what you’d expect here: practical, plucky, determined, and a bit pig-headed about being asked to be careful (even though it all turns out okay in the end, and of course that’s because she disobeys and goes off-script). Most of the other characters are bit-parts, even the love interest, though Lady Heritage is surprisingly vibrant — if anything she has more life than Jane through her depth of feeling, which is fairly apparent on several occasions. (By contrast Jane rather suddenly decides Henry is her darling, after previously having turned him down for what we presume are good reasons, given her usual practicality.)
It’s a bit of fun, and being written by a woman, it isn’t quite so bad as a lot of classic mysteries in the way it handles female characters. Jane’s a bit scathing about other girls for not having the same interests or bravery, but she and Lady Heritage aren’t a bit like the rather silly, inconstant girls of some of the mystery fiction written by men. (Hello, John Dickson Carr is calling!)