I read this only because I needed a Sci-Fi book for my library's book bingo, and I ended up really liking it! The Aliens are not really explained that well, and the 4th and 5th waves don't make a lot of sense. However, I was willing to look over that because the characters were well-developed and interesting. All of the characters face numerous moral dilemmas in a post-apocalyptic landscape. Cassie is trying to find her younger brother, Sammy. She carries around Bear, her brother's teddy bear that she promised to protect. Ben is just trying to keep himself alive after barely surviving the 3rd wave (a global pandemic). He cannot get over his guilt because he didn't protect his little sister. The book isn't a masterpiece, but it's an intense take on YA dystopian novels.