This is the precursor to all those French historical romances like the Three Musketeers! (... which is exactly why I picked up, to be clear.)
Vigny was reportedly more of a royalist, so here we have a version of the 17th century where Cardinal Richelieu is the most evil man to ever evil, ever. There he is, twirling his mustache with evil glee. There he goes, cackling maniacally while his adorable cats play in front of him. (I do like that Vigny preserved Richelieu's love of cats, that was nice.)
The story revolves around the titular Cinq-Mars (silent q, silent s, so it's pronounced "Saint Mar"), a young nobleman who witnesses the cruelty of Richelieu's reign when he journeys past the town of Loudun. There, he sees the witch trial of Urbain Grandier (tldr: he's a Catholic priest who (a) took advantage of some young female parishioners of his, not cool at all, (b) defied Richelieu's attempts to take away regional power, and (c) was ultimately burned at the stake in a horrific death that, at last for what they charged him with, he probably didn't deserve).
Anyway, Cinq-Mars sees this whole debacle (highly romanticized by Vigny) and then heads up to meet the king, whose favorite he quickly becomes. He decides he's going to be ambitious and pursue power (much to the chagrin of his forever-friend, de Thou, a/k/a the man who consistently tells Cinq-Mars how much he loves him, and to whom Cinq-Mars consistently replied "aw buddy, that's nice, you're my best friend too :)") (bold of Vigny to give us an explicitly Orestes-Pylades pairing where Pylades is trying to get out of the friend zone). And by the end, Cinq-Mars is heading up a coup to rid France of the cardinal and force the king to reclaim his rightful authority.
... It does not go well.
What I didn't love was Vigny's storytelling, to be honest. I thought his view of the historical conflict was too simple, too geared on a good vs evil binary (with maybe some neutral but ultimately selfish opportunists thrown for good measure). Vigny is here to say: nobility is where it's at, because a strong nobility means the king has equals who can help him reign, as opposed to a king with strong centralized power that can topple over at the first hint of weakness. This is the core of his beef with Cardinal Richelieu, whose influence did, in fact, help centralize the king's power. Vigny ignores a lot of the reasons behind that political strategy--namely, Louis XIII's brother and step-brothers and a few other nobles kept trying to overthrow him... kind of worth a mention, but what do I know? (And I haven't even covered Louis XIII's reasons for teaming up with Cardinal Richelieu, or how the two men mutually kept each other alive and could not survive without the other, especially at the start when they cooperated to get rid of Louis's mother, Marie de' Medici.)
Basically, I didn't love how Vigny adapted the politics to serve his plot. I know Alexandre Dumas does this, but where Dumas does a good job of portraying his royals, nobility, and ministers as "at the end of the day, their petty squabbles are for their own benefit, and they hold the lives of many innocent people in their hands like so many pawns on a chessboard," it feels like Vigny wants me to actually root for one side or the other. And I just don't feel like rooting for either side. So I found myself rooting for de Thou, and even he felt oddly devoid of agency, going along with Cinq-Mars despite being audibly upset with him and his terribly thought-out political plans.
What I did like was the ending! (No, not because it ended, I swear.) At the end, Cinq-Mars is trying to convince his beloved fiancee that he did all of these things for her, betrayed France to Spain for her, etc. and as he swears these things, he realizes he isn't certain of what compelled him to act. He realizes his ambition may have been the motivation all along, and not, as he initially claimed, the means to his more virtuous end of restoring power to the king by killing off the cardinal. And at the same time, there's an interesting situation with his fiancee, who realizes that she doesn't know this supposedly virtuous young man very well, and marrying him would not only be a huge mesalliance by social standards, but also a bad move overall. (Queen Anne of Austria steps in to say this quite a lot. I really liked those scenes, despite their being a little on the side of "jaded but well-meaning woman talks young girl out of running away with the love of her life").
Overall, I'm glad I read it, even if it was pretty slow-going. It reminded me of how much heavy lifting Vigny must have done with this book, laying out the foundation upon which Alexandre Dumas, 20 years later, would build his incredible Musketeer trilogy.
Recommended for fans of French history, 19th century historical takes on the 17th century, and Musketeer enthusiasts (given that this happens in more or less the same period, vaguely between the end of the The Three Musketeers but before Twenty Years After).