This post contains lots of spoilers!!
This book happens somewhat shortly (the timeline was a little unclear to me) after a plague apocalypse. Not my favorite kind of apocalypse, but whatever. Basically, just about everyone died of smallpox and then a new strain swept through the remaining survivors. So not many people are left.
Lucy has managed to survive alone. Her family all died in the plague. She has her own little camp set up and she's avoided people since she believes that most of them are dangerous. The S'ans are crazy, the sweepers will kidnap you, and the scavengers will steal from you. So she keeps to herself.
Then one day she happens upon a random boy, Aiden. He helps her escape dogs sent out by the sweepers. She thinks he's annoying, but intriguing, and she's grateful for his help. That night she awakes suddenly to a tsunami about to happen. She's barely able to escape with some of her belongings but her camp is completely destroyed.
She decides to make her way to where Aiden said his settlement was. She gets there just in time to see the sweepers come and take away a few people. The scavengers (who apparently are not evil as she'd feared) welcome her in. Henry is a little older than Lucy and flirts a lot. Pretty girl Del is obviously into Aiden and therefore sees Lucy as a threat. Leo is a bit older and obviously used to be in the military. And there's an older woman who takes on a grandmotherly role for everyone. There are few S'ans there too, who are apparently not evil either. S'ans have survived the virus, but it's left their skin blackened and their eyes red. She had heard that it infected their brain too, making them crazy. But that was obviously an urban myth. One of the S'ans is Aiden's brother, Sammy.
The sweepers come and attack again and steal Leo and Del. Lucy feels responsible and wants to rescue them, but everyone tells her it's not worth the risk. Then Del magically comes back - somehow she was able to escape when no one else ever has. And then Aiden finds Leo, who somehow has caught the plague and is about to die. He dies and then Del, Aiden, Lucy agree to rescue everyone else that has been caught over time.
The sweepers come from a hospital on an island. They are able to sneak in and once inside, they realize that Del sold them out. Apparently, the doctor is only after Lucy. There's something special about her. She was never vaccinated but she was somehow able to survive the plague. Aiden is pissed and Del tries to get him to leave with her and the kids she bargained for but he refuses. The doctor seems nice, but drugs Lucy (why when she could have just talked her way into helping her?). Then Sammy magically shows up to rescue them and Lucy steals her medical file from the doctor's office. Danger and adventure ensues and they are able to escape. And Aiden and Lucy are happily together in the epilogue.