O Pioneers! is a novel which tells the story of a family of Swedish immigrants, the Bergsons, who in rural Nebraska during the early 20th-century. The narrative focuses primarily on Alexandra Bergson, who is left in charge of the farm and her younger brothers after the death of her father, and on her younger brother Emil. The story follows Alexandra and Emil's romantic relationships and Alexandra's struggles to run the farm while exploring themes of isolation, temptation, and marriage.
The book is divided into five parts, each of which has numerous chapters.
On a windy January day in Hanover, Nebraska, Alexandra Bergson is with her five-year-old brother Emil, whose little kitten has climbed a telegraph pole and is afraid to come down. Alexandra asks her neighbor and friend Carl Linstrum to retrieve the kitten. Later, Alexandra finds Emil in the general store with Marie Tovesky. They are playing with the kitten. Marie lives in Omaha and is visiting her uncle Joe Tovesky.
Alexandra's father is dying, and it is his wish that she run the farm after he is gone. Alexandra and her brothers Oscar and Lou later visit Ivar, known as Crazy Ivar because of his unorthodox views. For instance, he sleeps in a hammock, believes in killing no living thing and goes barefoot summer and winter. But he is known for healing sick animals. Alexandra is concerned about their hogs as the hogs of many of their neighbors are dying. Crazy Ivar advises her to keep their hogs clean rather than letting them live in filth and to give them fresh, clean water and good food. This simply confirms Oscar's and Lou's opinion that Ivar deserves the name Crazy Ivar. Alexandra, however, starts making plans for where she will relocate the hogs.
After years of crop failure, many of the Bergson's neighbors are selling out, even if it means taking a loss. Then they learn the Lindstrums have also decided to leave. 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, Alexandra talks her brothers 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. That same day, the Bergsons are surprised by a visit from Carl Linstrum, whom they have not seen for thirteen years. [Note: Carl says it has been sixteen years, but this is a textual error. John Bergson died sixteen years earlier, and Carl's family left during the drought that occurred three years later. ] Having failed at a job in Chicago, he is on his way to Alaska, but decides to stay with Alexandra for a while. Carl notices the growing flirtatious relationship between Emil and Marie. Lou and Oscar suspect that Carl wants to marry Alexandra, and are resentful at the idea that Carl might try to marry into a farm, while they had to work hard for theirs. This causes problems between Alexandra and her brothers, and they stop speaking to each other. Carl, recognizing a problem, decides to leave for Alaska. At the same time, Emil announces he is leaving to travel through Mexico. Alexandra is left alone.
Alexandra spends the winter alone, except for occasional visits from Marie, whom she visits with Mrs. Lee, Lou's mother-in-law. She also has an increased number of the mysterious dreams she has had since girlhood. These dreams are about a strong, god-like male figure who carries her over the fields.
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 Emil and Marie realize what they value most. Before leaving for Michigan, Emil 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 in a drunken rage. Heflees to Omaha, where he later turns himself in for the crime. Ivar discovers Emil's abandoned horse, leading him to search for the boy and discover the bodies.
After Emil's death Alexandra is distraught, in shock, and slightly out of it. She goes 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 who reminds her of Emil, 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, telling her that he is back. They decide to marry, unconcerned with the approval of her brothers.