There's a great site
here where classic books are very consisely and wittily summarised. (There's a movie version, too!) But quelle surprise, The Scarlet Pimpernel is not amongst the ranks - perhaps because it might offend Jane Austen!
So. I thought us fans could make up our own.
How about:
Chauvelin
I am French, and therefore evil. Do as I say.
Sir Percy
I am British, well dressed and very, very rich.
Marguerite
I am French, but married to a rich Englishman. Therefore, I don't know what to do for the best.
(She betrays her husband because Chauvelin tells her to, but all is forgiven.)
Not the best example, but you get the idea! Your turn!