This is a tough review to write as part of the reason I enjoyed it was no one had spoiled it for me. I'll do my best not to spoil it for anyone else.
Nick and Amy move from Manhattan (her hometown) to Missouri (where he grew up) to help his sister care for his parents. Amy is initially portrayed as a diehard New York who is miserable in the Midwest but tries to make it work for his sake.
To outsiders, it seems like they have a good marriage but then Amy goes missing on their fifth anniversary. Nick is the primary suspect but he swears he's innocent. His attitude towards the police and the media make it seem like he's hiding something but we're not sure what.
The book goes back and forth between Nick's perspective and Amy's. Nick's being present time (and not dated), and Amy's dated and providing more of the backstory.
I almost gave up in the beginning as Nick's attitude was just bugging me, and I thought we would never learn anything substantial about the reason for it, then the author sucked me in. It was a very well planned out book, giving you little pieces of information along the way so everything is plausible even if there are some surprises.