How It All Goes Down
Like a lot of teenage girls, Cassie Sullivan's life is filled with ups and downs. One day her biggest problem is that her crush, Ben Parish, barely knows that she's alive. The next thing she knows her brother has been kidnapped and she's lying in a pit of human ash, watching her father get shot to death.
Oh, did we mention the aliens? They're the jerks that are ruining her life, not to mention the lives of the seven billion other people they murdered in cold blood.
Seven. Billion. People. That's pretty much everyone on the planet.
When we meet Cassie, she's been "enjoying" some time by herself in the wilderness, trying to regroup and strategize after all that drama. Occasionally, she makes supply runs or shoots a soldier in the head, but mostly she just writes in her journal.
We learn most of what happened to her in the first months after the alien invasion through flashbacks. The aliens are called the Others, and they've been attacking Earth in stages (or "waves").
Cassie's already survived waves one through three. First, the Others killed the electricity. (No biggie.) Wave Two involved giant tsunamis that took out coastal regions all over the world. (Sometimes it pays to live in the Midwest, folks.) Wave Three was horrifying pestilence that makes your eyes bleed. Yikes.
That last one is what killed her mom. Yeah, she's dead, too.
Back in the present day, Cassie's dealing with the Fourth Wave, which actually turns out pretty great because it involves hot alien assassin guys. One of them, Evan Walker, shoots Cassie in the leg, but he feels so bad about it that he takes her back to his farm to patch her up. His actions break all sorts of alien protocols, but that's love. What can you do?
As he and Cassie tell each other their life stories, Evan leaves out the parts about being an alien and shooting her in the leg. (Just meaningless details, really. Trifles.) Still, Cassie's Spidey-sense is tingling like crazy. As she and Evan plan a mission to rescue Sammy (her little brother), Cassie tries to ignore her strong suspicion that Evan is definitely an alien. She distracts herself by making out with him constantly. Not a bad plan, really.
Meanwhile… remember Ben, Cassie's high school crush? Somehow he's survived this whole mess, too. After almost dying from the alien pestilence, he's renamed himself Zombie and enrolled in a grueling boot camp at Camp Haven, a military base. And, hey, look at that—it just so happens that one of his fellow trainees is Sammy, Cassie's little brother.
Eventually, Zombie graduates from boot camp and is sent into battle. His smartest squad member, Ringer, figures out that their targets aren't actually aliens and that Zombie's superiors back at Camp Haven aren't who they're pretending to be. They aren't human; they're Others, and for secret reasons they're training human children to hunt and kill humans. Sorry, that's all the information on the alien agenda that we have at this time.
Zombie freaks out and devises a cockamamie plan to go rescue Sammy, who's back at the military base. Everyone in his squad thinks it's suicide, but he's determined to find a way because they were tight like that. He has Ringer shoot him in the stomach so he's evacuated back to Camp Haven. Then he takes advantage of the chaos created by a base-wide alarm to stagger through the building and look for Sammy.
When Zombie finally finds the little dude, Sammy's already getting rescued by Cassie, who beat him to the punch because she's awesome. (By the way, while Zombie was doing all that other stuff, Evan finally spilled his guts to Cassie about being a secret alien. She wasn't exactly happy about it, but she got past it.)
The three of them—Ben, Cassie, and Sammy—manage to escape while Evan blows up the building… and maybe himself. Whoops.