O Pioneers! follows the lives and fortunes of the Bergson family, Swedish immigrants with a farm in Hanover, Nebraska. Just in case you're wondering: no, there is no O Pioneers! theme park in Hanover. Too bad, right? In fact, it turns out Hanover is a made-up town, somewhere on the Nebraska prairie (known in the novel as "the Divide"). Cather invents this place in order to tell the coming-of-age story of Alexandra Bergson, the eldest of the Bergson children, who doggedly persists during the worst of times to become a wealthy, independent landowner.
Okay, now let's break it down. This is a novel in five parts, spanning about thirty years. John Bergson, the father of Alexandra, dies in the first part, leaving his wife and four children to fend for themselves in this tough and inhospitable land. Fortunately, it doesn't stay that way for long. By the second part of the novel, the Divide has transformed into a lush, populous country, and through her clever farming practices, Alexandra has made her family buckets of money.
Alexandra finds financial success, but she definitely has bad luck in the love department. Amidst tough conditions on the Divide, her childhood friend, Carl Linstrum, moves away to make his living in a city. And then, when he returns years later in Part II, he's driven off by Alexandra's younger brothers, Lou and Oscar, who suspect he's after their family fortune. Alexandra's plans to marry Carl are crushed.
Meanwhile, things are getting hot and heavy between Emil, Alexandra's favorite youngest brother, and Marie Shabata, the Bohemian beauty next door. But Marie's in a tough position; though her husband, Frank, is a jealous, embittered you-know-what, she still fears falling in love with Emil. Well, soon it's too late for that, and Emil decides the best thing for him is to get away. So, he heads down to Mexico.
Part III sees the coming of winter, and Marie's deepening depression about her failed marriage to Frank. But it's not until Part IV, when Emil returns from Mexico, that the real drama starts. First, Marie and Emil kiss at a Catholic fair at the French Church. In the drama that follows, Marie admits she loves him, and Emil agrees to leave for law school, never to see her again.
Not long afterward, though, Emil's friend Amédée collapses in the fields and dies from a bad case of appendicitis. And during the service for Amédée, under the influence of the church music ("Ave Maria"), Emil decides he can't leave Marie after all. He leaves the church in a frenzy, rides to the Shabatas' and finds Marie in the orchard. They embrace passionately, and… well, you can figure out the rest.
Still, you just know it can't end like that. Frank comes home later that night and Marie is nowhere to be found. Frank takes his rifle and goes out looking. When he hears a noise in the orchard, he shoots without stopping to think—he kills Emil and Marie. By Part V, Frank's serving time for murder in the state penitentiary.
Needless to say, Alexandra is devastated by Emil's murder. To get herself back on her feet, she decides to visit Frank in prison, finding him barely a shadow of his former self. Then, that same day, Alexandra receives a telegram from Carl: he's back in town. They make plans to get married, and Alexandra decides to go with Carl to Alaska. But as she contemplates leaving, she realizes that she'll always belong on the Divide. She figures she'll be back after the season is over.