The following version of the book was used to create this study guide: Soontornvat, Christina. The Last Mapmaker. Candlewick Press, Somerville, MA, 02144. 2022. Kindle AZW file.

Sai Mudawan is a 12-year-old girl who lives in the kingdom of Mangkon. She lives in a very poor neighborhood known as the Fens and works in the city of An Lung. The social structure of this region is based on family lineage. Those who are able to trace their family ancestry several generations exhibit that knowledge by wearing a lineal – a chain with one line for every named generation. The lineal is presented on a child's 13th birthday. Those with lineals have the best opportunities for education and jobs that lead to the best lifestyles. People like Sai who are too poor to afford a lineal or otherwise unable to track their ancestry have few chances to change their poverty-ridden existence. Sai happens to be in the right place at the right time to land a job with Master Paiyoon, the last mapmaker to be skilled in the traditional way of making maps. Paiyoon does not ask Sai directly about her lineal. Sai eagerly does her job with the knowledge that she will face unanswerable questions after she turns 13 years old. Sai lives in the Fens with her father, a man known only as Mud, and his friend Catfish. Mud does not hold a job and is often jailed for his criminal activities. Sai has been saving a portion of her salary each week with the intention of escaping her terrible home life.

When Paiyoon manages to secure a place on an exploration voyage, he discovers that the medication that stops the trembling in his hands will not last for the many months of the voyage. He is thrilled when Sai wants to go with him. Sai has no interest in the exploration. She plans to use her saved money to begin a new life in some distant land. Before Sai can leave, Mud discovers there is something amiss. Faced with the knowledge that the ship is about to leave without her, Sai gives Mud all of her savings. He is shocked into letting her go. On board the ship, Sai realizes she cannot start a new life without that money.

Life on the ship settles into a routine as Paiyoon teaches Sai about making maps. Sai befriends a woman named Rian who tells Sai they need to reach a mystical land called the Sunderlands. Paiyoon seems intent on keeping them from setting that course. Sai also befriends a young stowaway named Bo. Just when Sai discovers that Bo is the secret son of Captain Sangra, who is also related to the queen, Rian forces Paiyoon and many loyal to Sangra off the ship, taking over command herself. With Sai's mapmaking skills, Rian sets a course for the Sunderlands.

There is continued strife and more adventures as they reach the area. Sai discovers a sea that a dragon lives in the Sunderlands, and she knows that humans would be determined to kill the creature. Sai manages to take back control of the ship and returns to An Lung where she is reunited with Paiyoon and Bo is reunited with Sangra.