Mary lives in a small village surrounded by a fence which is designed to keep the Unconsecrated (zombies) out. The zombies appeared a few generations back and as far as the villagers know they're the only people left in the world. The ocean is regarded as a myth.
It's a tight knit community, ruled by the Sisterhood (basically nuns). Mary is sent to join them when no one speaks for her (asks to marry her).
Apparently women can either get married and breed or join the Sisterhood. For some reason women have only one chance at marriage which doesn't really make sense if repopulation is the priority.
She quickly finds that the Sisterhood has some big secrets.
There are intriguing hints about where the zombies came from, I'm hoping that will be fully explored in later books.
I'd also like to know why Gabrielle was so much faster than the other Unconsecrated.
It's an interesting story and setting, but it's hard to feel sympathy for Mary. She really is quite selfish and seems more than willing to sacrifice other people to get what she wants.