El Dorado by Baroness Emma Orczy

Eldorado, by Baroness Orczy is a sequel book to the classic adventure tale, The Scarlet Pimpernel. It was first published in 1913. The novel is notable in that it is the partial basis for most of the film treatments of the original book.
A French language version, translated and adapted by Charlotte and Marie-Louise Desroyses, was also produced under the title La Capture du Mouron Rouge.
As well as containing all the main characters from the first book, Eldorado introduces several new characters and features the Baron de Batz, who also turns up in Sir Percy Leads the Band and The Way of the Scarlet Pimpernel (Baron Jean de Batz is a genuine historical figure).
It is 1794 and Paris, "despite the horrors that had stained her walls - has remained a city of pleasure, and the knife of the guillotine did scarce descend more often than did the drop-scenes on the stage."
The plot begins when Sir Percy reluctantly agrees to take Armand St. Just with him to France as part of a plan to rescue the young Dauphin.