O Pioneers! tells the story of the Bergsons, a family of Swedish immigrants in the farm country near the fictional town of Hanover, Nebraska, at the turn of the twentieth century. The main character, Alexandra Bergson, inherits the family farmland when her father dies, and she devotes her life to making the farm a viable enterprise at a time when other immigrant families are giving up and leaving the prairie. The novel also follows two romantic relationships, one between Alexandra and family friend Carl Linstrum and another between Alexandra's brother Emil and the married Marie Shabata.
On a windy day in Hanover, Nebraska, Alexandra Bergson is with her five-year-old brother Emil. Emil's kitten has climbed a telegraph pole and is afraid to come down. Alexandra finds her neighbor and friend Carl Linstrum, who retrieves the kitten. In the general store, Alexandra finds Emil with Marie Tovesky who is two years older than Emil. Marie's father has brought her from Omaha. Alexandra's father is dying, and he wishes that she run the farm after he is gone. They later visit Crazy Ivar, who advises them to keep their hogs clean. When the Linstrums are leaving, Oscar and Lou want to leave too, but neither their mother nor Alexandra will. After visiting villages downwards to see how they are getting on, she talks her brothers Oscar and Lou into mortgaging the farm to buy more land, in hopes of ending up as rich landowners.
Sixteen years later, the farms are now prosperous. Alexandra and her brothers have divided up their inheritance, and Emil has just returned from college. The Linstrum farm has failed, and Marie, now married to Frank Shabata, has bought it. During a Bergson family get-together, Carl Linstrum shows up, having failed in a job in Chicago. He is on his way to Alaska, but decides to stay with Alexandra for a while. There is a growing flirtatious relationship between Emil and Marie, which Carl notices. Lou and Oscar suspect that Carl wants to marry Alexandra, and are resentful that they had to work hard for their farms, but he thinks he can marry into a farm. After this, Alexandra and her brothers are no longer speaking. Then Carl, recognizing a problem, decides to leave for Alaska. At the same time, Emil announces he is leaving for a job in Mexico City. Alexandra is left alone.
Emil returns from Mexico City. His best friend, Amédée, is now married with a young son. At a fair at the French church, Emil and Marie kiss for the first time. They later confess their illicit love, and Emil determines to leave for law school in Michigan. Before he leaves Amédée dies from a ruptured appendix, and as a result both he and Marie realize what they value most. Before leaving he stops by Marie's farm to say one last goodbye, and they fall into a passionate embrace beneath the white mulberry tree. They stay there for several hours, until Marie's husband, Frank, finds them, and shoots them. He goes off to Omaha. Ivar discovers Emil's abandoned horse, leading him to search for the boy and discover the bodies.
Alexandra has gone off in a rainstorm. Ivar goes looking for her and brings her back home, where she sleeps fitfully and dreams about death. She then decides to visit Frank in Lincoln where he is incarcerated. While in town she walks by Emil's university campus, comes upon a polite young man, and feels better. The next day she talks to Frank in prison. He is bedraggled and can barely speak properly, and she promises to do what she can to see him released; she bears no ill will toward him. She then receives a telegram from Carl, saying he is back. They decide to marry, unconcerned with her brothers' approval.
Even as a boy, Emil’s face has been imprinted by the land: “It was from facing this vast hardness that the boy’s mouth had become so bitter” (7). The novel takes its name from a line in Whitman’s Leaves of Grass, and the land plays a significant role in the novel. Unlike southern literature, in which there’s often a connection to the land, in this novel the land is almost another character, one who’s demanding, but conquerable to those who are willing to risk big and work hard.