Most good stories start with a fundamental list of ingredients: the initial situation, conflict, complication, climax, suspense, denouement, and conclusion. Great writers sometimes shake up the recipe and add some spice.
Exposition (Initial Situation)
So You Think You've Been Invaded by Aliens
Things haven't been going so well for the good citizens of planet Earth. First a huge alien ship appeared in the sky. Ten days later, murder and mayhem ensued. When we first meet Cassie, she thinks it's possible that she's the last person in the universe. She knows for sure that seven billion people are already dead. That's more than 99% of the world's population, for those of you playing along at home.
Rising Action (Conflict, Complication)
Sammy Kidnapped, Cassie's Dad Is shot, Cassie Kills The Crucifix Soldier
We stay with Cassie through the rising action, much of which is revealed through flashbacks. After the death of Cassie's mom, she and her family went to a refugee camp. There, the Others kidnapped Sammy and shot Cassie's dad. Since then, Cassie's been on her own… unless you count that guy she blew away with an M16 in the back of a convenience store. She's scared and she doesn't know who she can trust.
Climax (Crisis, Turning Point)
Huge Amounts of Drama
Cassie has a bit of a close call when a Silencer—that's her name for the aliens who are hunting survivors like her—shoots her in the leg. She survives for a few days, only to get stuck in a blizzard. She almost freezes to death, but instead she wakes up in a nice warm bed that belongs to a nice hot guy named Evan Walker. Score.
Meanwhile, we're introduced to a second protagonist: Ben Parish, Cassie's former classmate (and crush). Ben is having a health crisis, too, with a real nasty case of alien plague. (He bleeds out of his eyes and everything.) Somehow he survives and starts boot camp at a military base. Eventually he becomes an elite alien assassin.
Things come to a head when Cassie and Ben learn two life-changing facts. Cassie's now-boyfriend Evan is a covert alien, and so are Ben's superiors at the military base. Lines are drawn. Cassie decides to team up with Evan, and Ben goes rogue, abandoning his duties.
Falling Action
Things Are Sort of Looking Up?
Separately—neither knows the other is alive—Cassie and Ben plan to infiltrate the military base. Their mission: to save Sammy. (That's Cassie's little brother and Ben's former squad mate.) Despite truly terrible odds, Cassie and Ben manage the rescue, finally bumping into one another along the way. The three of them hitch a ride to safety courtesy of Ben's squad members. Meanwhile, Evan stays behind and blows the base to bits.
Resolution (Denouement)
Welcome to Cliffhanger City: Population You
This book is the first in a trilogy, so it makes sense that, once Sammy is rescued, there's not much in the way of tying up loose ends. When we hit the last page, we have no idea if Evan lived through the explosion—or if humanity itself will survive.