It is April 3rd, 1492. Brother Girolamo is a Dominican and the First Brother of San Marco in Florence. He can see and banish demons, as we find out in the first chapter when he cleanses the convent of Santa Lucia. The demons appear to be drawn by a green stone hidden in a hollowed-out copy of Pliny, a donation to the convent library from the King of Hungary. That green stone will be central to the story, but neither we nor Girolamo find out why for some time. The only hint is that the dying Lorenzo de' Medici implies that it is the stone of Titurel.
Brother Girolamo is also a prophet. He has the ability to see the future, sometimes explicitly and sometimes in symbolic terms. Sometimes the events can be changed, and sometimes they have the weight of certainty. He believes the New Cyrus will come over the Alps, leading to the sack and fall of Rome, and hopes to save Florence from the same fate by transforming it into the City of God.
If your knowledge of Italian Renaissance history is good, you may have already guessed the relevant history. The introduction of additional characters named Marsilio and Count Pico provide an additional clue before Walton mentions Brother Girolamo's last name: Savonarola.
If, like me, you haven't studied Italian history but still think this sounds vaguely familiar, that may be because Savonarola and his brief religious rule of Florence is a topic of Chapter VI of Niccolò Machiavelli's The Prince. Brother Girolamo in Walton's portrayal is not the reactionary religious fanatic he is more often shown as, but if you know this part of history, you'll find many events of the first part of the book familiar.
The rest of this book... that's where writing this review becomes difficult.
About 40% of the way through Lent, and well into spoiler territory, this becomes a very different book. Exactly how isn't something I can explain without ruining a substantial portion of the plot. That also makes it difficult to talk about what Walton is doing in this novel, and to some extent even to describe its genre. I'll try, but the result will be unsatisfyingly vague.
Lent is set in an alternate historical universe in which both theology and magic work roughly the way that 15th century Christianity thought that they worked. Demons are real, although most people can't see them. Prophecy is real in a sense, although that's a bit more complicated. When Savonarola says that Florence is besieged by demons, he means that demons are literally arrayed against the walls of the city and attempting to make their ways inside. Walton applies the concreteness of science with its discoverable rules and careful analysis to prophecy, spiritual warfare, and other aspects of theology that would be spoilers.
Using Savonarola as the sympathetic main character is a bold choice. The historical figure is normally portrayed as the sort of villain everyone, including Machiavelli, loves to hate. Walton's version of the character is still arguably a religious fanatic, but the layers behind why he is so deeply religious and what he is attempting to accomplish are deep and complex. He has a single-minded belief in a few core principles, and he's acting on the basis of prophecy that he believes completely (for more reasons than either he or the reader knows at first). But outside of those areas of uncompromising certainty, he's thoughtful and curious, befriends other thoughtful and curious people, supports philosophy, and has a deep sense of fairness and honesty. When he talks about reform of the church in Lent, he's both sincere and believable. (This would not survive a bonfire of the vanities that was a literal book burning, but Walton argues forcefully in an afterward that this popular belief contradicts accounts from primary sources.)
Lent starts as an engrossing piece of historical fiction, pulling me into the fictional thoughts of a figure I would not have expected to like nearly as much as I did. I was not at all bored by the relatively straightforward retelling of Italian history and would have happily read more of it. The shifting of gears partway through adds additional intriguing depth, and it's fun to play what-if with medieval theology and explore the implications of all of it being literally true.
The ending, unfortunately, I thought was less successful, mostly due to pacing. Story progress slows in a way that has an important effect on Savonarola, but starts to feel a touch tedious. Then, Walton makes a bit too fast of a pivot between despair and success and didn't give me quite enough emotional foundation for the resolution. She also dropped me off the end of the book more abruptly than I wanted. I'm not sure how she could have possibly continued beyond the ending, to be fair, but still, I wanted to know what would happen in the next chapter (and the theology would have been delightfully subversive). But this is also the sort of book that's exceedingly hard to end.
I would call Lent more intriguing than fully successful, but I enjoyed reading it despite not having much inherent interest in Florence, Renaissance theology, or this part of Italian history. If any of those topics attracts you more than it does me, I suspect you will find this book worth reading.