At the beginning of the novel, Ken has again angered his father by returning home from boarding school with failing grades. This means he will have to repeat a grade – an expense Rob can ill afford. Nonetheless, Nell eventually persuades Rob to let Ken choose a colt of his own. Ken looks over the available young horses, unable to decide; then he sees a golden sorrel filly running swiftly away from him one day, and makes his choice.
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. All the Goose Bar horses with the same strain have been fast, beautiful, but utterly untameable, and after many years of trying to break one of them, Rob has decided to get rid of them all. Ken persists, however, and Rob reluctantly agrees to let him have the filly. When Rob and Ken go out to capture her, she lives up to her family reputation: she tries to escape by leaping an impossibly high barbed wire fence and injures herself severely.
This gives Ken an opportunity to tame the filly by gently nursing her back to health. He names her Flicka – Swedish for 'little girl' – and spends hours every day tending to her needs and keeping her company. Flicka gradually comes to love and trust the boy; however, her barbed wire wounds cause a dangerous blood infection and she begins to waste away. She grows so thin and weak that Rob decides that 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, stumbles, falls, and is unable to rise. Ken finds her there and spends the rest of the night sitting in the water, holding her head in his arms. Although Ken nearly dies from exposure, the cold running water cures Flicka's fever, and all ends well.