Exposition (Initial Situation): I'm Gonna Be Great!
Carol Milford begins this book attending college and dreaming great dreams about how she's going to change the world. But that all hits a snag when she marries Dr. Will Kennicott and moves with him to his dull rural town of Gopher Prairie. Carol tries at first to force her ambitions onto the town, but she meets a lot of resistance from the people there. She wants them all to change and improve themselves, but they're happy to continue on doing things as they've always done them.
Rising Action (Conflict, Complication): Not So Fast!
The more Carol pushes the people of Gopher Prairie to change, the more they push back and tell her to be satisfied with things the way they are. Carol responds by distancing herself from her husband Will and going for nature walks as much as she can. She even strikes up an emotional affair with a young tailor's assistant named Erik Valbourg. Like Carol, Erik has big dreams, so it makes sense that Carol thinks he's the only person she can truly connect with.
Eventually, Will Kennicott finds out about the affair and puts a stop to it. Carol realizes that she can't go back to her old life, so she takes her son Hugh and moves to Washington, D.C. to get away from her husband and Gopher Prairie. She ends up living there for two years.
Climax (Crisis, Turning Point): Huh? What Happened?
We'll be real: Main Street doesn't really have a climax in the normal sense. Yes, Will Kennicott comes to Washington to see if Carol will come home with him. But after a sober talk, they both decide that Carol needs a little more time to find herself.
When Carol finally makes the choice to head back to Gopher Prairie, she does it more out of fatigue and resignation than anything else. It's not like she has a "Eureka!" moment, or anything; she just realizes that she's experienced everything she's ever going to experience in Washington. Oh, yeah—she also finds out she's pregnant again, and the timing of the pregnancy doesn't exactly suggest that the baby is her husband's.
Falling Action: Back to the Grind
When Carol moves back to Gopher Prairie, she's actually kind of happy to see her old neighbors, but she also knows this feeling will get old quickly. It's not totally clear what would make Carol happy; it just seems like she has an ambition that could only be truly fulfilled if she were a man. Unfortunately, her options are limited by the fact that she's a woman in 1920s America.
Resolution (Denouement): In the Year 2000
Now that she's back living with Will, Carol wants to make a few things clear. She tells Will that even though she's back, she'll never be satisfied with Gopher Prairie or its people the way they are. She admits that she hasn't really changed anything, but she'll also never let go of her dissatisfaction. To which Will responds: "Yeah, yeah. That's great, Carol. Say, do you think the maid left the furnace on?"
So, yeah—Carol can look forward to much more of the same life that drove her away in the first place. Her one great hope now is that her daughter will grow up to have opportunities that Carol herself never had.