The moral/lesson of the book is along the lines of one's chickens coming home to roost- sooner or later.
Sad because the 'mistake' that the h is accused of by one and all is the one thing I feel she should be applauded for as it involves standing up against the benevolent tyranny of her elders and to run away to escape marriage to a man she thinks of as a brother. Of course, in those times running away without a maid or companion - and money, into the unknown, is fraught with more than one kind of danger (even when disguised as an unattractive woman) but the biggest, it seems, is recognition and thus, social ruin!
On the way, she meets the H who's looking for his equally (if not more) irresponsible sister, and by a stroke of fate and the girl's whims finds 'employment' as a maid to the H's sister. The H is mighty suspicious of this not very subservient maid and keeps asking her for references which she keeps fobbing off by making bad excuses or by losing her temper. These interactions are interesting and funny and quite believable actually.
As a maid, she finds life not exactly pleasant and gets embroiled in the below stairs politics and at the receiving end of advances of a footman as well as her employer.
Midway, she escapes to be a (newly returned to London) cousin and the book dips somewhat. The expected makeover and come-out happens but the h's fear of recognition casts a pall over the goings on and I was hard put to accept that no one recognizes her. The author stretches out the story till the expected pre-climactic 'exposure' and then the wrapping up.
But the exposure takes more than one unexpected turn and other people also get affected. So, the almost moralizing tone (not intentional) and the overstressed burden of honor and ruin on a woman's life gets a tad wearing.
And the bad guys get let off too easily. One should have been transported and the other given the glove slap!