In the early 1900s of the Wild West, Milagro Navarro-Prescott (Eiza González) performs her trademark act in the circus until she falls from a horse during one of her performances and dies. Years later, her young daughter, Fortuna Esperanza "Lucky" Navarro-Prescott (Isabela Merced), has been living with her aunt Cora (Julianne Moore) and grandfather James (Joe Hart) in a city on the east coast. However, after Lucky lets a squirrel in their house and ruins her grandfather's campaign party (James is a railroad baron, who is now running for GOvernor of the state), she and Cora head to the western town of Miradero (James wont speak to Fortuna and asks Cora to take her away) where they will stay with Lucky's father Jim (Jake Gyllenhaal), whom Lucky has not really known of after the incident with Milagro, for the summer. On the train over, Lucky notices a mustang running with his herd, and goes to the back of the train to meet him. Upon arriving in Miradero, the pair meet local young riders Pru Granger (Marsai Martin) (who is a rodeo rider) (and her father Al (Andre Braugher)), and Abigail Stone (Mckenna Grace) (and her brother Snips (Lucian Perez), who has a donkey named Carrots). Abigail has a horse named Boomerang. Fortuna is awkward with Jim, since she cant figure out why Jim abandoned her after the death of Milgaro. Fortuna finds her mother's old room in Jim's house.The next day, Lucky goes to a corral where she sees the same horse from before trying to be abusively manhandled by a man named Hendricks (Walton Goggins). Hendricks is an outlaw and had seen the mustang with his herd from the train (he was on the same train as Fortuna). Lucky tells Jim and Cora about the horse, thinking he can be her new friend, but Jim does not want Lucky going near the horses after what happened to Milagro. Nonetheless, Pru and Abigail help Lucky win over the trust of the horse (Lucky entices the mustang with apples), whom she names Spirit. After a while, Spirit trusts Lucky enough to eat an apple she offers him, and allows her to pet him, sealing their bond.Lucky attempts to ride Spirit but he escapes from the corral and runs away, while Lucky is saved by Pru and Abigail after almost falling off a cliff. Jim finds and scolds Lucky for disobeying him. They go home arguing, making Lucky think he has given up on her. She sneaks out at night through the forest where she finds Spirit and his herd together, but are interrupted by Hendricks and his men, who capture all the horses but Spirit (who resists fiercely and is left alone by Hendricks) and take them in with a stolen train. Knowing Hendricks and his men are wranglers, who are taking the horses to a boat to be auctioned and worked to death, Lucky agrees to help Spirit rescue his herd. She enlists Pru and Abigail with their horses to embark on a journey to try and cut the wranglers off on their path by beating them to a water depot located beyond a summit called Heck Mountain (which is near impossible to cross on horseback, but allows them to shortcut the train to the docks, where Hendricks will load the horses on a boat). During the trip, Lucky builds up her relationship with Spirit, and becomes good friends with Pru and Abigail as they travel. Their plan is to reach a water depot, where the train has to stop to refill its water, before Hendricks gets there and ambush the train.The girls reach the water depot the night before the wranglers are set to arrive, but Lucky wakes up the next day realizing the wranglers made it earlier than expected. Spirit desperately attempts to save his herd on his own, but gets taken in by the wranglers. Determined to save Spirit, Lucky and her friends follow the train to the docks, where the wranglers are boarding the horses onto the boat. The girls arrive too late to stop the boat from leaving, but they manage to save Spirit. Lucky and Spirit get on board the boat by leaping off the docks, and free the other horses and subdue the wranglers, with Spirit knocking Hendricks off the boat. Jim, Cora, and Al (who were notified of the girls absence by Snips) show up on a fast steam train (JIm had been building the fastest train in his shed and brought it out to chase down Hendricks), and arrest Hendricks and the other wranglers. All the horses and Lucky jump off the boat and reach the shore.On their way back to Miradero, the girls travel to a field in the middle of the wilderness, and Lucky lets Spirit go, saying that he needs to stay with his herd. When she returns home, Lucky and Cora initially plan to return to the city, but after Lucky bonds with Jim at a rodeo, she decides instead to stay in Miradero with him and her friends, as Spirit is seen riding through the fields with his herd.