From The Chronicles of Narnia Wiki

The Horse and His Boy takes place in Calormen and Archenland, neighboring countries south of Narnia, during the reign of Peter, Susan, Edmund, and Lucy. It is the only book which focuses on main characters from the Narnian world, instead of Earth.

A young boy, Shasta, living in Calormen with a poor fisherman, Arsheesh, runs away from home with a talking horse of Narnia, Bree, when he finds out Arsheesh is not his father and is going to sell him into slavery. They decide to escape north to Narnia. Along the way they join up with a girl Aravis, a high-born Tarkheena running away from a forced marriage, and her talking horse Hwin.

However, in the capitol city Tashbaan, Shasta is mistaken temporarily for young Prince Corin of Archenland, whom he resembles, who is visiting Calormen under the care of a Narnian delegation headed by Queen Susan and King Edmund. Before he can escape, Shasta learns that Susan and Edmund are planning to leave by night in their ship Splendor Hyaline to escape Prince Rabadash, son of the Tisroc, who is in love with Queen Susan. Meanwhile Aravis, who has sneaked into the Tisroc's palace to get help from her friend Lasaraleen, accidently overhears a conversation between Prince Rabadash and his father the Tisroc. Furious that Susan has escaped him, Rabadash gets permission to invade Archenland, so that he can conquer Narnia and capture Queen Susan. The two girls almost get caught by Aravis's prospective husband, Ahoshta, who is Grand Vizier.

Reunited, the four travellers decide to ride to Anvard in Archenland to warn King Lune about the invasion. After a harrowing three day journey across the desert just ahead of Rabadash's invasion force, they meet the Hermit of the Southern March who urges Shasta on alone. After getting lost in the mountains, Shasta meets up with the King in a hunting party, and warns him of the invasion. They return to Anvard in time to defend it from Rabadash's force, and Shasta fights in the battle. Aided by the arrival of a defending force from Narnia under King Edmund and Queen Lucy, they are victorious. The unrepentant Rabadash is captured, and given a donkey's head by Aslan as punishment for his aggression. Afterward, the resemblance between Shasta and Corin is discovered. It turns out that Shasta is really Corin's twin brother Prince Cor, who was lost at sea as a baby and found by Arsheesh, and King Lune is his father. Shasta (or Cor) returns for Aravis, who has stayed at the hermit's house, and invites her to stay with him at Anvard. She does, and they eventually marry. Prince Cor will become king, and their son will become King Ram the Great.