This story, all four books in the quartet, is about the making of a hero. It is also about a very stubborn girl.
Alanna of Trebond wants to be a knight of the realm of Tortall, in a time when girls are forbidden to be warriors. Rather than give up her dream, she and her brother--who wants to be a mage, not a knight--switch places. She becomes Alan; Thom becomes a student wizard in the school where she would have learned to be a lady.
The quartet is about her struggle to achieve her goals and to master weapons, combat, polite behavior, her magic, her temper, and even her own heart. It is about friendships--with the heir to the throne, the King of Thieves, a wise and kindly knight--and her long struggle against a powerful enemy mage.
She sees battle as a squire and as a knight, lives among desert people and tries to rescue an independent princess. Singled out by a goddess, accompanied by a semi-divine cat with firm opinions, somehow she survives her many adventures to become a most unlikely legend.