This is very much a Scarlet Pimpernel novel in disguise. The setting is France, but France twenty years after the Pimpernel, during Napoleon's abortive return to power up to and including Waterloo. An Englishman who is not what he claims to be (Bobby Clyffurde) moves between the sides aiding the anti-Revolutionary cause as best he can. As with Percy Blakeney before he was shackled with a wife, Clyffurde is pursuing a romantic passion as well as political aims.
I'm thinking this may actually have been a more comfortable period than the Revolution for Orczy to work in. Bobby's spy work against Napoleon (he's on the payroll of the British government, as opposed to Percy's amateur status) allies naturally with his national interests and does not involve any slavish attachment to the fortunes of the spoiled and haughty French émigrés. In the Pimpernel novels, there was always a bit of tension between on the one hand the narrator's conventional historical stance that the French aristos were a deplorable lot who brought much of their fate upon themselves, and on the other the wholehearted dedication of Percy and his band to rescuing those same aristos as victims. In this novel, while there is some ambivalence about Napoleon himself - Orczy again echoes her own contemporaries in admiring his "genius" - there is none about his movement, which was against English interests and therefore deserved all-out opposition from Clyffurde. So, while Clyffurde does his share of aristo-rescuing, his motives are less pro-aristo than anti-Bonaparte. They are, of course, especially pro-Crystal, his ineffable blonde French-born English-raised love, for whom he make the expected hyper-honourable sacrifices up to and including rescuing the less than honourable aristocratic rival he believes her to be in love with.
Crystal herself is a classically maddening Orczy female. She has both complete deference to her family's interests and a spontaneous spunkiness when enforcing ideas of "honour" on her trio of suitors (the third is a Bonapartiste, exposed on their affiancing day). Amongst the suitors only Clyfforde has the same sort of ideals as she does, including the bizarre habit of working against one's own self-interest as if it were some sort of virtue in itself to do so.
By contrast, there is an elderly aunt figure (or fairy godmother), full of common sense and good works, whose only failure as a character is that she seems always just a little too infallibly right about what everyone is thinking, doing and about to do. She is entirely necessary to the happy ending for the lovers, who otherwise would no doubt have contented themselves with a single swooning rose-perfumed waltz at the ball before Waterloo.
Baroness Orczy had clearly done her homework about the Battle of Waterloo and wanted to show it off. The battle section of the book is therefore about two chapters too long, though the incidents involving her fictional characters are fairly well distributed. She did write one very striking scene of Napoleon grimly leading his horse back in the direction of the battle, not to take up arms but to find his peers and plot how to use the swings on the London Stock Exchange, consequent on changing reports of the battle's outcome, to rebuild his fortunes!
I'm not sure why I keep coming back to these romances of the first decades of the twentieth century, when those of the twenty-first century leave me cold or worse. Perhaps it's just that Baroness Orczy writes very good fairy tales that do not pretend to be more, and I can appreciate them as such.