I would first like to apologize because I haven't read the book so I can't say weather it goes along that story line.
SPOILERS!!!Only some :)
The movie was pretty good in my opinion. The beginning was different which I personally liked. The beginning was a prologue of how it began. The aliens came and the waves started.
Wave One: Darkness, the electricity was wiped out. The water system, the internet, and lights.
Wave Two; Destruction, floods, tsunamis, and earthquakes.
Wave Three : Disease, they had the birds spread the disease through the world killing billions.
Wave Four: They walked on Earth, disguised as humans.
Wave Five: This wave was when the aliens were impersonating the military and taking children and training them to kill the remaining humans that they tricked the children into thinking they were the aliens.
The main character through this was Cassie who is a normal teenager who loves her brother dearly. And when he is taken and her dad is murdered she sets off to find her brother and keep him safe because he is all she has left. In her new world she has to adapt to romance seems like the least of her worries, until it literally comes face to face with Evan Walker. At first she is wary of this stranger but she has to choice but to trust him if she wants to find her brother.
Through the movie you go back and forth between Cassie and her brother Sam who is being trained to fight were he encounters Ben who is Cassie's crush from high school. While Cassie is becoming more and more "involved" with Evan. When Cassie finally finds her brother she also discovers a secret about Evan. Meanwhile Sam and Ben discover whats truly going on the military base. Together they have to figure out how to escape.