What is The Horse and His Boy about?

The Horse And His Boy is part of the Chronicles of Narnia series of books by C.S. Lewis. It is the fifth book of the series.
This book takes place during a time when the Pevensie children (Lucy, Susan, Peter, and Edmund) are kings and queens of Narnia. By contrast, in the book The Lion, The Witch, and The Wardrobe, the Pevensie children have only just discovered Narnia. 
The protagonist of this book is Shasta. He is a Narnian boy, but he has been raised as the son of a fisherman in Calormen. The man he knows as his father plots to sell him into slavery to a Tarkaan (which is what the noblemen of Calormen are called.) Shasta begins talking to the Tarkaan's horse, who is a Narnian horse. Together, they decide to run away and return to Narnia. 
Shasta and the horse meet Aravis on their journey. She and her horse (Hwin) are running away to avoid the marriage her parents are trying to arrange for her.
On the journey, Shasta discovers he is a doppelganger for the Prince of Archenland, a kingdom that sits between Calormen and Narnia. In Narnia, Shasta meets Queen Susan who is being courted by a prince. Aravis learns that the prince is planning a secret attack on Archenland and Narnia, so Aravis and Shasta team up to try to warn the King of Archenland about the plot. They meet up with Aslan along the way, who has been guiding and protecting them. They join up with the King of Archenland for a battle against the prince and his people. Shasta finds out that he is actually the son of a king who had been lost long ago, and Shasta will now one day rule a kingdom.