Shasta is running away from an abusive home life. A Narnian boy, Shasta has been raised in the Southern land of Calormen by a fisherman called Arsheesh, who treats him abominably. When Shasta gets wind that the man he calls his father is planning to sell him into slavery, he realizes that the time has come for him to leave—and fast.
The fisherman plans to sell Shasta to a complete stranger. As one can imagine, the young boy's worried about kind of person this stranger will be. It's bad enough being a slave, but it's even worse to be a slave to someone who's cruel and uncaring. In fact, slavery is the best word to characterize Shasta's experience of living with Arsheesh. But it could always be much worse. And the last thing he wants is to jump from the frying pan into the fire, as it were.
So he takes off, accepting the invitation of the stranger's horse to come with him to the fabled land of Narnia, which is the boy's ancestral homeland. Just before he leaves, Shasta discovers that he was found by Arsheesh as a baby in a boat on the beach, alongside a man who was dead. This confirms that Arsheesh isn't his father and that he doesn't really belong in Calormen. There's no longer any good reason for Shasta to stay.