You may have noticed that Foundation isn't really a novel in the sense that it tells one complete story. It's actually five different stories sharing a fictional history, all of which happen to be bundled beneath the same cover. Ergo (Latin!), each of the five stories has its own plot structure.
We're going to take a stab at the three-act plot of Foundation's first story,"The Psychohistorians." This should help you recognize the three-act plot structure at work in the other four stories. Go ahead and try it out yourself on another story—you might be surprised what you learn.
The first act of any story sets up the conflict. We learn (1) who the characters are, (2) where the story's set, (3) what the special story rules are, (4) what the major conflict is, and (5) what the stakes of the conflict are.
"The Psychohistorians" barrels through all of these points in its first act, and that act consists of chapters 1-4. Yep, half of the entire story.
In Act 1, we learn about Gaal Dornick's desire to work with Hari Seldon (Characters).Through Gaal's eyes,we wander through the planet of Trantor and get a grip on this technology savvy future (Setting). We also meet Hari Seldon for the first time, and the great mathematician explains psychohistory (Special Rules). Seldon also predicts the fall of the Galactic Empire (Conflict). However, using psychohistory, he hopes to limit the inevitable dark age from a bleak 30,000 years to a relatively minor 1,000 years (Stakes).
And voila! Simply by reading the story, you've learned all the information you need to read the story. Cool, huh?
Those in the story-making biz call Act II the "rising action." That's because this part of the story should raise the tension. It accomplishes this by having the characters try to resolve the conflict introduced in Act I.
However, in trying to solve the conflict, the characters often enter a worse situation, forcing them to deal with a new set of problems on top of everything else. If done right, the tension will be nail-bitingly great.
Act II of "The Psychohistorians" begins with Chapter 5, when Gaal is arrested. Seldon, Seldon's team, and Gaal are all put on trial for treason. If they can't prove their innocence, they'll be executed. If that happens, Seldon will be unable to accomplish his plan to help the galaxy in the ensuing crisis (death has a tendency to interrupt plans).
The tension is further bumped up by psychohistory. Seldon's calculations say that everything should work out in his favor, and he's a smart cookie, so he should know. But Gaal and the reader are left wondering whether or not psychohistory can predict the future or not. After all, we haven't seen it in action yet.
Guess we'll have to read on to Act III to find out.
The name of Act III's game is resolution. We discover whether or not the characters succeeded in overcoming the obstacles put in their way, and all lingering questions should be answered. (Well, maybe not all the questions. Sometimes questions are left unanswered.)
In Chapter 7, the Committee gives Seldon and Gaal a choice: death or exile. Seldon chooses exile (obviously), and it seems like he has been defeated. But Seldon reveals that all has actually gone according to his plan. He wanted to be exiled to the ends of the galaxy, so he could build two scientific refuges: the Foundations. These two refuges will shorten the inevitable Dark Age making Seldon's plan work out perfectly.
Conflict, resolved. We do have one lingering question, however. What happens next? Will the Foundation succeed and how? To answer that, we'll have to read "The Encyclopedists" and start analyzing the three-act plot structure all over again.