Alanna and her twin Thom are about to be sent off to the medieval-fantasy-equivalent of boarding school by their emotionally distant father. Tomboy Alanna is supposed to go learn magic and how to be ladylike, while bookworm Thom is supposed to train to become a knight. Lightbulb time: they decide to switch places! Alanna cuts her hair and rides off with the Trebond family's head soldier, Coram, while Thom goes with Maude, the healing-woman, to train in sorcery.
Bullied into going along with Alanna's ruse, Coram brings "Alan" to the capital city of Tortall, Corus. There, she reports to Duke Gareth of Naxen, who's kind of sponsoring her at court. Then Alanna meets the other boys in her age group, including Ralon (a bully), Gary (her first BFF), and Jonathan (the crown prince).
Now that our merry band of stock characters is established, we get down to the hard part: being a page. And boy is it hard. Alanna attends classes every morning and warrior training every afternoon. Since she's small, she ends up bruised and exhausted pretty much every day—but not too exhausted to befriend Sir Myles, an eccentric historian knight; and George Cooper, king of the thieves of the capital city. (Thief-king: good guy to have on your side.)
Ralon continues to bully Alanna, going as far as breaking her arm in a scuffle. Alanna asks George to teach her hand-to-hand fighting, and after training on her own for a while, she calls out Ralon, insulting his parentage and honor … and then she gives him such a whooping that he leaves court and never returns. And we're not sorry.
After this mini-climax, it's same-old, same-old at the palace until the Sweating Fever strikes, incapacitating and killing people all over the capital city, with some bizarre magical force draining the healers who try to tend the victims. Prince Jonathan falls ill, and even though Alanna is normally afraid to try to use her magic skills, she knows she has to save him. She channels some kind of a mysterious, feminine power that reclaims Jonathan from the hands of death. Weird, right? But also super convenient.
In Alanna's second year as a page, Jonathan's cousin, Duke Roger, comes back to the palace. He's a powerful sorcerer who gives Alanna the willies, so she tries to conceal her magical gift from him. Other highlights of her second year include George acquiring a beautiful horse for her, Alanna getting her period (and being royally ticked off about it!), learning that she sucks at swordfighting, and spending a ton of extra time practicing on her own.
We finish things up with two journeys. First, Sir Myles brings Alanna to his estate, confessing that he was compelled by dreams to do so. When they visit some ancient ruins, Alanna finds a magical sword in a threatening storm and must accept the possibility of her own death in order to take the sword and leave the ruins. Then, Alanna accompanies Jonathan and some of the other squires to Persopolis, a desert city. Strangely, Duke Roger warned Jonathan to stay away from the nearby Black City—but the warning sounded more like a dare, so obviously Jonathan and Alanna go check it out.
Turns out (drumroll please) the Black City is eeeeevil. A bunch of ancient forces live there and devour the souls of anyone who wanders in. Yummy! Since Alanna and Jonathan both have magic abilities, they manage to (just barely) defeat the Nameless Ones, with the help of Alanna's trusty magical sword. During the fight, Alanna's real identity gets exposed, so she has to explain her story to Jonathan. He accepts her just as she is and even asks her to be his squire. Awesome!