The first four waves have wiped out nearly all of humanity. The true humans are in hiding for their lives. Think of something along the lines of The Host type scenario going on. 
 Cassie has been separated from her younger brother Sammy but will stop at nothing to get him back. She saw The Others ride off with him and tons of other kids on the back of a bus being taken somewhere. She thought she could trust the army, but the army has betrayed her and been infiltrated. 
 Zombie (view spoiler)[aka Ben Cassie's school boy crush! (hide spoiler)] is in training to kill The Others. Zombie as well as a ton of others are being trained on how to combat and stop the enemy from taking over Earth. 
 I love how this story is told from multiple points of views of people. I wasn't expecting that at all going into reading this book and it really adds a nice little twist to it! 
 (view spoiler)[ 
 Infestations were embedded in key individuals around the world, sleeper agents, if you will, waitinng for the signal to launch a coordinated attack after our population had been whittled down to a manageable number. We know how that attack turned out here at Camp Haven, and we strongly suspect that other military installations were not as fortunate. 
 I fell in LOVE with Evan aka "the other" who saved her life. Who was going to stop at nothing to protect his own humanity he had discovered as well as Cassie's. He helped Cassie in the end get to Camp Haven where her brother was being held with tons of other children as assassins for "the others" secretly. Cassie saves her brother along with the help of Ben Parrish to get out of Camp Haven before Evan makes it collapse. 
 Only downfall was we lost Evan along the way! Please please let him be alive and come back. Ben doesn't even recognize Cassie for who she is really, and didn't even recognize her when they were in school together! Evan has recognized her all along and is giving up his own race for her. 
 The fifth wave has started and it's using children as an army against the human race. 
 (hide spoiler)]