There is a town within a valley. To the west is an identical town in an identical valley, but 20 years in the past. To the east is an identical town in an identical valley, but 20 years in the future. There are fences and guards preventing anyone from crossing the borders without approval from the Conseil, a branch of the government that manages the interactions between the valleys. One can request to visit another valley as part of their grieving process after the loss of a loved one, for example, but the request may be approved or denied by the Conseil based on how likely the requestor is to interfere with the timeline of the other valley. Those visiting other valleys must wear masks hiding their identities and not interact with residents, to prevent disastrous ripple effects throughout their world.

Odile is a quiet and bullied 16-year-old when she happens to see and identify visitors from another valley. They're the parents of a boy in her class, Edme, come to catch a glimpse of him in her valley, which Odile knows means his death is imminent. She reveals her knowledge to her teacher, which leads to her being sworn to secrecy but landing the opportunity to compete for a Conseil apprenticeship. On track for the life she's always wanted, Odile should be thrilled. But she finds herself growing closer and closer to Edme, and more conflicted than ever. There are laws for a reason, and while leaving Edme to his fate feels wrong, altering the timeline could have even worse consequences.

I don't want to give too much away besides the synopsis because man this book blew my mind. On the surface, it's such a simple concept, but the more you think about it and the deeper you get into the story, the more you see the potential ripple effects of a world like this, and it's fascinating. I loved the exploration of the logistics and mechanics of the world, and the way it made me think about fate vs. free will. If you grew up loving 2010s dystopian books this might hit just the right spot in your adult brain, like it did for me. The only thing I will flag here is that this book doesn't use quotation marks for dialogue. It really confused me at first, and if I'd known that before I might not have requested the ARC. Once the story got going and my brain got used to it I didn't mind as much as I feared I would, but definitely a bizarre choice. I think it's worth the read anyway, but brace yourself. Thank you to NetGalley, the author, and the publisher for the advance copy in exchange for my honest review.