Shasta is a boy who lives in the southern part of the country of Calormen. His adoptive father, Arsheesh, is a poor fisherman who found Shasta on a boat when Shasta was an infant. One day, a Tarkaan (lord) rides into Shasta’s small town and starts negotiating with Arsheesh to purchase Shasta and enslave him. While this negotiation is happening, Shasta comes upon the Tarkaan’s horse and discovers to his surprise that the horse can talk. Bree, the Talking Horse, warns Shasta that the Tarkaan is a cruel man, and together, the two of them sneak away, heading north toward Narnia, where Bree promises people live freely without slavery.
At one point in their journey, Shasta and Bree get chased by lions. In their efforts to get away, they run into Aravis, a Tarkheena (Calormen noblewoman) who is trying to escape an unwanted marriage, and her Talking Horse, Hwin. The four of them decide to travel together.
When the group reaches the city of Tashbaan, Shasta, Aravis, and their horses all decide to disguise themselves as peasants to avoid detection in the city crowds. They make a plan to meet on the far side of the city if they get separated, and sure enough, Shasta gets pulled away from the others by some men while they’re all attempting to get through the city. It turns out that several Narnians have mistaken Shasta for Corin, a prince who looks almost exactly like Shasta. Shasta keeps up the ruse as he meets Queen Susan and King Edmund, two of the four sibling monarchs of Narnia. The monarchs have become trapped in Tashbaan, where the local Prince Rabadash intends to marry Susan, by force if necessary. Eventually, the real Corin returns to take back his place, Shasta makes his way to the meeting place, and Susan and Edmund escape the city by boat.
Meanwhile, Aravis has a chance meeting with her friend Lasaraleen, and over the course of their conversation Aravis realizes how different she has become from many of the other Calormenes, even her old friends. She finally gets away to rejoin Shasta by the Tombs to the north of the city.
After discovering Susan’s escape, Prince Rabadash declares to his father, the Tisroc (emperor) of Calormen, that he will ride across the desert into Narnia with 200 men and take Susan back. The Tisroc allows him to go, so long as he pretends to be acting independently, without the Tisroc’s approval. Rabadash’s plan is to start with a siege of Anvard, the castle where King Lune, the king of Archenland, lives. When Shasta and Aravis learn of this plan, they set off at once to try to reach King Lune and warn him.
During their rush to find King Lune, the party gets attacked by a lion. Although Bree behaves cowardly and gallops away to save himself, Shasta bravely turns back to stop the lion from attacking Aravis and Hwin. They all go to a Hermit who helps tend to Aravis and Hwin’s wounds. Shasta continues his journey alone and reaches King Lune, where he explains the coming attack. Shasta then rides with King Lune back to Anvard on an unfamiliar horse, but the horse is slow, and Shasta gets left behind in a dense fog. In the fog, Shasta meets a large invisible creature who reveals himself to be the Talking Lion Aslan. Aslan says he has been watching over Shasta’s journey the whole time. When the fog clears and Aslan is gone, Shasta has apparently crossed through a mountain pass into Narnia without realizing it.
In Narnia, Shasta meets up with a party that includes Edmund, Lucy, and Corin, who are all traveling toward Anvard to stop Rabadash’s attack. A great battle ensures (which the Hermit watches from a distance), and with the help of Shasta and the Narnians, the army of Archenland totally defeats Rabadash and his army.
After the battle, everyone debates what to do with the captured Rabadash. Aslan appears again and curses Rabadash by turning him into a donkey. He tells Rabadash that he must travel back to a temple in his homeland, and if he ever strays too far from that temple, he’ll turn back into a donkey forever. Rabadash spends the rest of his life in Calormen and develops a reputation for being unintelligent.
Meanwhile, Shasta learns that the reason why he looks so much like Corin is that he’s actually Corin’s twin brother (and King Lune’s son), Cor. Cor was kidnapped and set adrift at sea as a baby as part of a conspiracy. Now, Cor returns to his rightful place as the heir to King Lune’s throne. He marries Aravis and goes on to have a successful reign after the death of his father. Bree and Hwin settle in Narnia, but they continue to come back frequently to visit Cor and Aravis in Archenland.
