The Horse and His Boy is one of the later books in the Chronicles of Narnia; it tells the story of Shasta, a fisherman's adopted son, who runs away from home with a talking horse, Bree. As the two make their way through the oppressive land of Calormene to Narnia, they meet the runaway princess Aravis and discover a conspiracy that may threaten the free lands to the north. The novel explores the themes of identity and Christianity.
A boy by the name of Shasta is found as a baby and raised by Arsheesh, a Calormene fisherman. As the story begins, Shasta overhears Arsheesh agreeing to sell him to a powerful Calormene feudal nobleman, Anradin. He is relieved to discover that Arsheesh is not his real father, since there was little love between them. While Shasta awaits his new master in the stable, Bree, the nobleman's stallion, astounds Shasta by speaking to him. He is a talking horse from Narnia who was captured by the Calormenes as a foal. He tells Shasta that Anradin will treat him cruelly, and Shasta resolves to escape. The horse suggests that they escape a life of servitude by riding north together to Narnia. They meet another pair of escaping travellers, Aravis, a young Calormene aristocrat, and her talking horse, Hwin. Aravis is fleeing to avoid a forced marriage with Ahoshta, the Tisroc's grand vizier.
The four must travel through Tashbaan, the bustling capital of Calormen. There they encounter a procession of visiting Narnian royalty, who mistake Shasta for Corin, a prince of Archenland, who was separated from their group earlier that day. Unsure what to do, Shasta goes with the Narnians and overhears their plans to escape from Calormen to prevent a forced marriage of Queen Susan with the Tisroc's son, Rabadash. Shasta escapes when the real Prince Corin returns.
Meanwhile, Aravis has been spotted by her friend Lasaraleen. She asks Lasaraleen not to betray her, and to help her escape from Tashbaan. Lasaraleen cannot understand why Aravis would want to abandon the life of a Calormene noblewoman or refuse marriage with Ahoshta, but she helps Aravis escape through the palace. On the way, they hide when the Tisroc, Rabadash, and Ahoshta approach. Aravis overhears the Tisroc and Rabadash discussing the Narnians' escape. Rabadash is still determined to have Queen Susan and wants to invade Narnia to seize her. The Tisroc gives Rabadash permission to invade Archenland and Narnia while High King Peter is preoccupied battling giants to the north.
Aravis rejoins Shasta and the horses outside Tashbaan, and tells them of the plot. The four set out across the desert, and a lion (whom they later discover to be Aslan) frightens them into fleeing swiftly enough to outrun Rabadash's army. Shasta arrives in Archenland in time to warn Archenland and Narnia of the approaching Calormenes. When Rabadash and his army arrive at the castle of King Lune in Archenland, they find their prey waiting for them, and a battle ensues. There is no clear outcome until an army from Narnia, led by Edmund and Lucy, reinforces the defenders. The Calormenes are defeated, and Rabadash is captured. Anradin is among those who fall in the battle.
Rabadash rebuffs King Lune's offer of conditional release, and is transformed by Aslan into a donkey. His true form will be restored if he stands before the altar of Tash at the Autumn Feast. However, he will become a donkey permanently if he ever strays thereafter more than ten miles from the Temple of Tash. For this reason, Rabadash pursues peaceful policies when he becomes Tisroc, as he dare not risk the ten mile limit by going to war.
The victorious King Lune recognizes Shasta as Cor, the long-lost identical twin of Prince Corin and, as barely the elder of the two, the heir to the throne. He was kidnapped as a baby to counter a prophecy that he would one day save Archenland from its greatest peril, but Shasta's timely warning has fulfilled the prophecy. Aravis and Shasta live in Archenland thereafter and eventually marry. Their son, Ram, becomes the most famous king of Archenland.