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
Leaving Here
Miles Halter, an introverted nerdtastic junior in high school, goes looking for a Great Perhaps (a great, uncertain future) away from home at Culver Creek Boarding School in Alabama. That's all we really need to know before diving into the action of the story.
Note: There are really two plots to the book with two rising actions and two climaxes. These are divided into Before and After.
Rising Action
Conflict Before: With a Little Help from My Friends
Miles arrives at school and tries to learn the social order, make some good friends, and pass his difficult classes. It's not an easy situation to be in, but we root for Miles as he struggles. His acquaintances with the Colonel, Alaska, and Takumi eventually blossom into friendships, and together they make mischief (in the form of pranks and smoking cigarettes) and drink a little too much.
Complication Before: Too Much on His Plate
Even when he gets his bearings at the Creek, Miles has got some troubles: the classes are way harder than he's used to, he's trying to figure out how to make and keep friends, he sort of likes Lara but he really likes Alaska, he needs to learn all the unwritten rules of the social order at the Creek, and he decides to adopt the us-versus-them mentality that the Colonel and Alaska have towards the Weekday Warriors. Phew—that's a lot.
Conflict After: Whodunnit?
It's not really a whodunnit, it's a what did the who do (and by the who we mean Alaska, not these guys). Miles and the Colonel are consumed with Alaska's death and what happened to her—did she commit suicide or was her death an accident? It's not an easy question to answer, but Miles and the Colonel feel especially driven to find out.
Complication After: Too Much of a Different Kind
Alaska's not alive to tell her friends what she was thinking right before she died, so they have to figure it out on their own. Oh—and Miles and the Colonel have this tremendous grief and guilt to cope with because they think they helped her to her death. This makes their coping with her death and the continuation of life at the Creek really difficult because they ignore classes (and their grades plummet), they ignore their friends, and they fight with each other. Not an easy thing to get through.
Climax
The First Climax: The Approaching Curve
Alaska drives away from campus, crying and drunk. On the highway, she sees a jackknifed semi and a cop car with its lights and siren on, and she plows right into the cop car. She dies instantly. Totally unexpected, this changes everything about the story.
The Second Climax: You Can't Always Get What You Want
After finding out that Alaska was headed to her mother's grave from Takumi, Miles and the Colonel realize that solving the mystery of Alaska's death—whether it was an accident or suicide—won't bring her back, so they finally let her go. It sort of seems like an anticlimactic climax, but in letting Alaska go, Miles and the Colonel accept her death and the fact that life moves on. It's this sort of catharsis (personal realization) that leads to a climax that's very different from the climax of Alaska's death.
Falling Action
The Climaxes Converge: Back to Life, Back to Reality
The two different climaxes—Alaska's death and Miles and the Colonel accepting her death—settle down in the same way: with Miles and the Colonel refocusing on school and trying to bring their grades up. It's not exciting stuff, but it is a way for them to enter back into the real world.
Resolution
But Miles Isn't Dead
So what? We still get to read his last words on the matter. Miles gets a letter from Takumi that admits he too could have stopped Alaska the night of her death, but just like Miles and the Colonel, didn't. Miles finally finishes his religion final for the Old Man and forgives himself and Alaska. This forgiveness is the real moment of closure for Miles, and we breathe a collective sigh of relief for him.