The Horse and His Boy was the fifth Narnia book C.S. Lewis wrote. In this book, he tells a story that took place concurrently with the time that the Pevensies (Peter, Susan, Edmund, and Lucy) were kings and queens of Narnia. The main character is Shasta, a Narnian boy who has been raised as the son of a fisherman in Calormen. When he hears that his father is planning to sell him into slavery to a wealthy Calormen noble (called a Taarkaan), he confers with the Taarkaan's horse who is a captured talking horse from Narnia. The two decide to run away and head north for Narnia. The adventures ensue.
Along the way they are meet up with a young runaway Tarkheena (noblewoman) named Aravis and her Narnian horse Hwin. Aravis is running away to avoid a marriage of state that her parents have arranged for her. Along their journey they have several adventures as Shasta finds he is an exact double for the Prince of Archenland, a small country located between Narnia and Calormen. When Shasta is mistaken for Prince Corin, he is brought to the palace where he learns that Queen Susan is visiting Calormen to be wooed by a Prince. But she has refused him and he does not want to let her leave. However she is planning to leave secretly. Aravis also learns that the Prince is planning a secret attack on Archenland and Narnia through the desert. When Aravis and Shasta join up again, they manage to cross the desert in a race against time to warn the King of Archenland about the attack. Just as they are getting too tired to continue on their way they are attacked by a lion and chased to Archenland where they stop at a hermit's cottage. Shasta goes on alone to find King Lune to warn him of the attack. He gets lost on the way there and receives guidance from Aslan, who he finds has been protecting him the entire time, including leading him into meeting up with Aravis in the first place. It was also Aslan who chased them to give them that last push to Archenland. Shasta warns the king in time for him to gather an army to defeat the invasion. As the battle comes to an end, Shasta finds that he is in actuality the long lost son of King Lune and twin to Prince Corin, who he has been told throughout the story was his double. Indeed he is the older twin, so Shasta will some day be king. Aravis decides to remain at the court of Archenland and eventually she and Shasta marry and reign over the kingdom together.
Quotes from The Horse and His Boy by C.S. Lewis