Shasta works for his father, Arsheesh the Calormene fisherman, who is somewhat cruel to him. A man comes by on a horse one day and offers to buy Shasta. Shasta overhears the men talking about how Shasta is not the fisherman's son. Shasta goes to talk to the man's horse, wishing the horse could tell him what his new master would be like. Unexpectedly, the horse does talk, introducing himself as Bree. He explains that he is a talking horse from Narnia and that the two of them may have a chance of escape if they do it together.
During their journey, they are chased by a lion. They are joined by a princess, Arivis, and her talking horse, Hwin, who are also being chased by a lion. After the lions disappear, they swap stories. Arivis is fleeing a forced marriage to the Tisroc's (king of Calormene) grand vizier. They decide that they may be less conspicuous if they travel together.
They decide that the best way is to go through the city of Tashbaan. They agree that if they should get separated, they would meet at the tombs on the other side. While there, Shasta is taken away by King Edmund of Narnia, thinking that he is Prince Corin of Archenland. He is taken care of by the Narnians and hears their troubles. Prince Rabadash, the Tisroc's son, wants to marry Queen Susan of Narnia. They believe that his intent is to start a war with Narnia and Shasta hears their escape plans. While Shasta is left alone, Corin returns. Shasta tells him what happened and quickly makes his escape after Corin tells him to tell his father that he is a friend of his.
Meanwhile, Arivis meets a friend, Lasaraleen, who is married to a great Lord. She agrees to hide Arivis and help her and the horses escape the city. In the process of sneaking out through a secret gate, Arivis and Lasaraleen overhear Prince Rabadash talking with the Tisroc. The plan is to take over Archenland quickly, while they are not prepared. From there, they will take over Narnia. If something were to go wrong, the Tisroc would say that he knew nothing of his son's plans.
Shasta is the first one at the tombs and has to spend a night there by himself. There is a cat that leads him to a spot where he will be able to sleep while waiting for the others. The next night, the group is reunited. They start heading in the direction that Shasta overheard from the Narnians. While trying to outrun Prince Rabadash's army of 200 men, they are attacked again by a lion. One of them scratches Arivis and Shasta has to jump off of Bree, who is too scared to stop, to save her and Hwin. A hermit appears and offers help. He sends Shasta alone to warn King Lune and the rest of Archenland. He finds them on the road and warns them in time. They hurry back to Archenland, but Shasta is lost in a fog. Arivis finds that her scratches were not that bad and Bree realizes that he will not be a famous war horse upon arriving in Narnia.
Shasta finds himself in Narnia talking to a hedgehog. Other animals appear and Shasta is able to send a message about the upcoming war along with a stag to Cair Paravel. Aslan appears to Shasta to explain how he has been protecting him his entire life. Shasta eventually meets up with King Edmund, Queen Lucy, and Prince Corin as they are on their way to Archenland to help King Lune defend his kingdom. Corin has an argument with a dwarf about being allowed in the battle. The dwarf ends up being hurt and Corin gives Shasta his armor. They charge into battle against the attacking Calormenes. Arivis is worried about Shasta, but is at least able to watch how Shasta is doing through the hermit's magic pool.
After the battle, the king approaches Corin and Shasta, berating them for joining the battle. However, he recognizes that this is what he would have done as well. He recognizes that Shasta is really Cor, is long lost son.
Aslan appears to the two talking horses and Arivis. He tells Bree that his fault is pride. Aslan explains that he gave Arivis the scratch on her back for her servant that was blamed for her escape. Cor (Shasta) goes to retrieve Arivis and tells her that the king would like her to live with them as well. Arivis agrees, happily. Upon their return to the kingdom, it is revealed to Cor that his is to be king because he is actually 20 minutes older than Corin. This makes Cor nervous, but makes Corin extremely happy. Cor and Arivis eventually get married so that they can argue and make up all the time.
The Chronicles of Narnia:
The Lion, The Witch, and the Wardrobe
The Voyage of the Dawn Treader
The Silver Chair
The Horse and His Boy
The Magician's Nephew
The Last Battle