Kenneth McLaughlin is a 10-year-old boy living on Goose Bar Ranch, just out of Cheyenne, Wyoming, with his practical father, Rob; his mother, Nell; and his older brother, Howard. Rob is often unsatisfied with Ken, who daydreams when he should be attending to practical matters; Nell, however, shares her son's sensitive nature and is more sympathetic. Howard, the older son, who looks and acts more like Rob, was allowed to choose and train a colt from among the Goose Bar herd (thoroughbreds that Rob McLaughlin is raising in the old-fashioned way, on the open range), much to the jealousy of Ken. Although Ken loves horses, Rob doesn't think his wool-gathering son deserves such a privilege yet.
At the beginning of the novel, Ken has angered his father by returning home from boarding school with failing grades and will have to repeat fifth grade, an expense Rob can ill afford. After a few mishaps in his first few days home, Nell convinces Rob to give Ken a colt, saying it will give him something to work towards and improve himself and his sense of responsibility. Ken is unable to decide which yearling he wants until he sees a beautiful sorrel filly running swiftly away from him.
Rob, once again, is annoyed with his son; this particular filly has a strain of mustang blood that makes her very wild– "loco", in ranch idiom. The mustang blood comes from a wild horse called the Albino, named for his pure white coat. All the Goose Bar horses with this strain have been fast and beautiful, but untameable, and after many years of trying to break just one of them, Rob has decided to get rid of themall. Ken persists, however, and Rob reluctantly agrees to let him have the filly. Rob, Ken, and the ranch hands make two attempts to capture her. During the second attempt, she tries to escape by attempting to jump an impossibly high barbed wire fence and injures herself severely.
Ken spends the rest of the summer nursing the filly. He names her Flicka– Swedish for "little girl" – and spends hours every day tending to her needs and keeping her company. Flicka comes to love and trust the boy, but her wounds fester and cause a dangerous blood infection. She grows so thin and weak that Rob decides she must be shot to put her out of her misery. The night before the order is to be carried out, Flicka wades into a shallow brook, falls, and is unable to rise. Ken sneaks from the house and spends the night sitting on the bank of the stream, with his legs in the cold water, holding her head to prevent her from drowning. When he is found the nextmorning, he has developed a high fever.
As the days pass, Ken's fever turns to pneumonia, and his condition continues downhill. On the other hand, Flicka gains strength steadily. After almost three weeks, Ken's condition improves, although since he believes Flicka is dead, he has no interest in the world. Rob takes Ken for a drive, and they see a stag protecting a doe. Ken realizes that like the stag's responsibility is his doe, his is Flicka. The book ends with Ken running out to see Flicka, who has fully recovered.