Exposition (Initial Situation)
Meet Amory Blaine, a spoiled rich kid from Minnesota whose mom thinks he's the greatest thing since sliced bread. The problem is that as Amory grows up, he must learn how to hide his sense of superiority if he's going to fit in (no one wants a friend who's a Veruca Salt clone, right?).
And this hiding takes a whole lot of effort. Eventually, he realizes how much his mother has limited his perspective and decides to enroll in a prep school in Connecticut. That way, he'll have a chance to develop his own personality away from his doting mommy.
Rising Action (Conflict, Complication)
Prep school and university give Amory a lot of freedom. But with this freedom comes the anxiety of wondering what he should do with his freedom. It's not long into university that Amory's grades start to suffer and he gets depressed about how he's going to make his life meaningful.
He can't fully get behind religion, and he's not motivated enough to put in the work it takes to be a huge financial or political success. Eventually, he turns to love as a path to fulfillment, but ends up getting his heart broken by a girl named Rosalind. She won't marry him because—yup—he hasn't worked hard enough to make himself rich. It's all a vicious cycle, really.
Climax (Crisis, Turning Point)
After months of binge drinking, Amory finds himself locked inside a hotel room where his friend Alec is about to be busted for hiring a prostitute. In an act of heroism, Amory takes the blame and gets his name published in the paper as a no-good john. Luckily, his mother and mentor have already died, so there's no one left for him to disappoint. But this climax is really an anticlimax, since it doesn't really change the aimless course that Amory is already on.
In the novel's final scene, Amory gets into a random rich dude's car to catch a lift to Princeton. He soon gets into a heated argument about socialism and the need for America's upper classes to come crashing down. The rich dude agrees with much of what he says, but not all of it.
Before getting dropped off, Amory realizes that the rich man's son was someone he (Amory) knew from Princeton. The son has since died in the war, so Amory and the man end up bonding over the boy's memory. The rich man ends the discussion by wishing Amory (but not his political views) good luck.
Once outside the rich man's car, Amory looks at the world around him and wonders what he'll do with his life. Years of wandering and wondering haven't led him to any satisfying conclusions. The only thing he really knows is himself, but it seems like this isn't good enough for him. He still needs something outside himself to give him meaning, but he can't figure out what it is.
Yup—if you're looking for an ending that's tied up neatly with a pretty bow, look elsewhere. No easy messages in This Side of Paradise.