Tris's Book, a fantasy novel by Tamora Pierce, tells the story of four young mages as they battle pirates and become closer than ever.
Lady Sandrilene fa Toren, Trisana Chandler, Daja Kisubo, and Briar Moss have been living in Discipline Cottage at Winding Circle a Temple in Emelan for some time now. They have survived an earthquake and are now having to defend their only true home from pirates. They use their magic, along with the help of their teachers, to try to drive off the pirates.
As the book begins and progresses, Tris hears voices on the wind telling her seemingly useless bits of information. In an effort to cool herself off in the summer's cruel heat, she and her three "mates"- Sandry, Daja, and Briar- head up to the wall that encloses Winding Circle. On the wall, Tris hears more voices and then suddenly a lighthouse nearby blows up. The four know that the two are related. As Tris and her teacher Niklaren Goldeye examine the ruins, they realize that the tower was blown up by a new substance they have never heard of. Also in the ruins she finds a baby starling, which Tris decides to take care of.
While repairing a magical net protecting Winding Circle with Dedicate Frostpine and his apprentice Kirel, Daja sees a magically hidden boat out of the corner of her eye. While eating a turnover from Dedicate Gorse's kitchen's, Briar smells the unmistakable scent of cinnamon and poppy, a sure sign of someone invisible. A forgetful novice realizes that there are a lack of bandages, so Dedicate Lark and Sandry are forced to make some more. The Temple is short on medicine, so Dedicate Rosethorn and Briar are forced to make more. All the signs lead to trouble. When Daja finally tells Frostpine about the ship, he immediately tells Dedicate Superior Moonstream, who sends word to Duke Vedris IV, Sandry's uncle and the ruler of Emelan. Pirates are afoot.
Tris's cousin Aymery Glassfire comes to bring Niko a letter, and is recognized by Tris. He tries to get Tris to go home, saying her father is ill. She refuses. While everyone is talking, a novice comes and tells Niko that every mirror, crystal, and vision bowl has been broken. That leaves Winding Circle blind to the future. Early the next morning, when jumping into a wind magically, Tris realizes that there is a huge invisible magic box coming slowly toward Winding Circle. Later in the morning, Frostpine takes Daja to repair two chains that protect Winding Circle. During the repairs, the pirates begin their attack.
As the attack goes on, the four start to help some. Tris takes one of the pirate's "boomstones"-- a hard sphere containing gunpowder-- with her winds and sends it away. Briar and Rosethorn take seeds of thorny plants and grow them quickly and thickly along the cove to capture and kill the pirates. When all the adults leave to help defend Winding Circle, Tris practices zapping targets with her lightning and the help of Sandry, Daja, and Tris. Tris tests herself when a boomstone begins a descent on Discipline and she blows it up.
When looking through Aymery's belongings, Briar finds that even though he claims he's staying for several weeks, Aymery has only brought a very small wardrobe. In a locked trunk, Briar finds cinnamon oil and poppy, used for invisibility. He also finds a mirror, when all of the mirrors in Winding Circle had broken the night before. When following Aymery out of Discipline at midnight, Tris and Briar confront him, whereapon he reveals that he had been working for Enahar, the pirates' chief mage, for some time. Then Aymery lets in the pirates through a side gate. Sandry, Daja, Lark, and Niko come and deal with the pirates, and Tris takes the dying Aymery's earring that links him to Enahar.
Maddened by helplessness, Tris goes up on the south wall to deal with the pirates herself and get them away from the one place she can call home. Sandry, Daja, and Briar join her and help. They bash through the pirates' barrier by pulling their strength together. Daja finds all the metal in the pirates's fleet like nails and weapons, pulls it out, and drops it in the water. Tris uses her lightning to blast ships. The four start destroying the fleet when they are all caught in mage traps on the ships. They appear to be lost when Niko returns their string of combined magic. With the extra power of the string and their arrived teachers, they destroy the rest of the fleet.
Because she killed and injured many of the pirates, Tris decides to help with the wounded captives. She gets little thanks. All she cares about is the safety of her friends and family.