Trickster's Choice opens with Aly returning to her home in Tortall after a sojourn. Her father, George of Pirate's Swoop, the second-in-command of spies, greets her. From her first appearance on the page, Aly is playful, precocious, and witty. She is also brilliant. Having been taught the tradecraft of a master spy by her father from the time she was a little girl, Aly, now sixteen, is eager to become an agent in the service of the king. Her father scoffs, believing the life of a spy is not suitable for his little girl. Her mother Alanna the Lioness—King's Champion, legendary warrior, and the first female knight in the kingdom of Tortall—reacts similarly upon her return from the battlefront in Scanra.
Alanna's visit to Pirate's Swoop marks the first time Aly has seen her mother in a year. Alanna and George cannot imagine why their daughter would want to be a spy. Instead, they believe her stated desire is a smokescreen masking her lack of ambition. Aly is frustrated and disheartened by her parents' wariness; she decides to set out on her own for several weeks to give her parents some time alone before her mother is called back to the war.
Aly's fortunes take a turn for the worse when the ship on which she sails is caught by pirates, and she is sold into slavery. After weeks in a holding pen, Aly is sold to the house of Duke Mequen Balitang in Rajmuat on Kypriang Island. Luckily, the duke and his family are amiable royalty who treat their slaves and servants with dignity and respect.
Bad news comes to the Balitang family by way of Prince Bronau from the Copper Isles. The family is forced to leave their home in Rajmuat because the ailing King Oron believes the duke is plotting against him. The family must move to a distant land holding. Aly is selected by the Balitangs to go with the family rather than be put back on the slave market because, as Aly finds out, the god Kyprioth has chosen her to help protect the family from harm. Kyprioth offers Aly a wager: if she protects the family, then he will tell her father of her abilities as a spy. Aly accepts the agreement, and along with the family, moves to the Lombyn.
Aly begins to suspect the true nature of her challenge along the road to Tanair. The island is inhabited by the copper-skinned raka people, who are deeply wary of the pale-skinned luarin because of the luarin history of oppressing the raka. Aly, a luarin, does not know what to make of the animosity between the two groups. The Balitangs are luarin as well, but the duke's two daughters—Dove, an inquisitive twelve-year-old, and Sarai, a beautiful sixteen-year-old—are half-raka. While Mequen is their father, their mother was Sarugani, a raka queen to whom the duke was married. Sarugani died prematurely in a riding accident. As the caravan proceeds on the road to their new home, Aly notices the raka people taking great interest in the girls. Although she is initially concerned, the raka are merely fascinated by the half-raka Sarai and Dove.
Duke Mequen and Duchess Winnamine have discovered that Aly is the god's messenger. They offer to remove the metal collar from around her neck and free her from slavery. Aly, however, knowing her responsibility to watch over the family, decides it would be beneficial for her to maintain the disguise of a slave. According to Aly, she can move more freely throughout the castle and the lands as a slave because very few people will suspect her of grander schemes. She is assigned to carry messages for the duke and duchess at night, and to herd goats during the day; this gives her full access to both the castle and the territory surrounding it.
As they settle into their new environment, Sarai and Dove frequently come to Aly to hear stories about Tortall. They want to know about her family; rather than tell them that she is daughter of the famed Lioness, she says her mother is a traveling musician. The girls are very interested in the Lioness, and Aly realizes that Dove, the younger of the two sisters, is as curious and intelligent as her sister Sarai is beautiful. Aly is also befriended by Nawat, a crow who has transformed himself into a boy. Nawat teaches Aly to speak the crow's language and shows her how to use the crows as allies in her quest to protect Dove and Sarai. In time, Aly and Nawat become good friends, and Nawat practices kissing with Aly as he has seen other humans do.
Kyprioth comes to Aly and, in her dreams, shows her scenes from home. In one, Aly sees her mother positioned at her military post. Alanna has learned that Aly is missing, and is angry with George for not telling her sooner. Her mother's concern is frightening to Aly, because she knows her mother needs to concentrate on her preparations for war. One slight misstep and her mother could be injured or killed. Aly suddenly worries that she might have endangered her mother by leaving home.
Aly maintains her cover as a slave until she discovers four raka servants meeting and discussing a conspiracy. She reveals herself to Ulasim, Chenaol, Fesgao, and Lokeij and learns the true nature of her charge. She is meant to protect Dove and Sarai, not the duke and duchess. The girls are raka royalty, and must someday reclaim their throne.
Aly and the raka conspirators decide that it is best for Aly to be armed. Aly has been thoroughly trained in daggers, so Chenaol, the head cook at Tanair, obtains a set of knives for her. Aly meets Junai Dodeka, a mysterious raka woman who appears to be following her. She learns that Junai is the daughter of Ulasim, and is a fierce warrior. Aly is unsure of why Junai is watching her, but assumes she is also part of the team of raka protecting Dove and Sarai. As Aly herds the goats, she is met by Dove, who has come to see her and discuss her mistrust of Prince Bronau. Although the prince is attempting to make inroads with the family under the guise of courting Sarai, Dove believes he has scandalous ulterior motives.
Aly rides with Dove, Sarai, Prince Bronau, and their servants into the countryside in Pohon. The people of Pohon are known to have a deep-seated hatred of the luarin because of the luarin's oppressive regimes. To protect the Balitangs, Aly believes they must find a mage that can avert any magical attacks. The Mage of Pohon is known to be powerful and wise. She is also known to hate the luarin fiercely.
Aly disguises herself by darkening her skin, and slips away from the riding party to find the mage. She is attacked in the village, but to her surprise, Junai...