The fey arrange Blackthorn's escape from prison and imminent death on the conditions that she use her skill only for good, that she help anybody who asks, and that she not seek vengeance against the man who destroyed her life. Accompanied by Grim, another escaped prisoner, she does as the fey said and travels north to Dalriada. They find a home their in the land of a young prince, soon to be married, whose fiancee has undergone an inexplicable transformation.
This first Blackthorn and Grim book has many elements in common with Marillier's other books: it's fantastic old Ireland/Scotland setting, its incorporation of folklore. It differs from her other series in that it's more a mashup of historical fantasy and mystery than of historical fantasy and romance: the story is very much about figuring out what happened to Lady Flidais, not about Prince Oran's and her relationship, and if there's to be any romance between Blackthorn and Grim, it's not in this book.
As for the basics of what happened to Lady Flidais, that can be seen a mile away, but finding out how the characters will find out and what detail will allow it to be undone is engrossing.
There is an ongoing condemnation of men in power who use it to abuse women that I found appealing, however, there's also an occasional unfortunate leaning towards slut-shaming. I would probably be more bothered by it if I hadn't read the rest of Marillier's books and known that there were quite a few of them with examples of characters "anticipating their wedding night" (which seems to have been her favorite term for it for quite some time, as I remember) without it being considered a major failing. It's definitely presented as one quite frequently in this book. But because I have read, for example, Son of the Shadows, I stop and consider that a certain character who is described as being a bit wild is not condemned for that reason by anyone. Everybody seems both to assume that Blackthorn and Grim are lovers and to remain unshocked by that. The argument one character makes that a young woman who was imprisoned and raped might have encouraged the man because she had another sweetheart is clearly presented as the odious victim-blaming that it is. All of the characters for whom chastity until marriage is presented as an essential quality are young men and women for whom said marriage is an important part of securing their future; perhaps it's more a matter of depicting what would have been a practical necessity. In any case, there's enough wiggle room for me to overlook it in a way I might not from an author with whom I was less familiar.
The summary of Tower of Thorns sounds like the series will continue in its unique historical fantasy mystery blend, and I eagerly look forward to it.