Sai Aravan, a 12-year-old girl, resides in the Mangkon kingdom. She serves as an assistant to Paiyoon Wongyai, a renowned mapmaker. Sai aims for success like Paiyoon but hides her commoner background. Despite her professional ambitions, Sai grapples with personal challenges. Her father, Mud, is abusive, and she resides in a poverty-stricken neighborhood.
In the Mangkon kingdom, only nobles can hold government positions unless granted the Lineal of Honor by the queen. After the conquest of the islands, the queen seeks ambitious individuals to explore potential annexations in new territories. Despite his desire to participate, Paiyoon cannot draw maps due to arthritis and old age. He proposes that Sai work under his name, presenting her with an opportunity to escape her abusive father for good. Despite Sai's efforts to leave, her father discovers her plan and attempts to thwart her departure by demanding all her savings in exchange for her freedom.
Desperate to escape, Sai reluctantly agrees to his terms. During her voyage on the ship Prosperity, Sai encounters Captain Anchalee Sangra and Miss Rian. Over time, she forms a friendship with Miss Rian and looks up to her as an idol. Through their conversations, Sai learns that the true purpose of the voyage is to locate the Sunderlands, rumored to be inhabited by dragons and abundant in valuable resources. Rian urges Sai to persuade Paiyoon to request the Captain to search for the Sunderlands since he does not wish to take the risk. Despite Sai's efforts, Paiyoon denies the existence of the island. Undeterred, Rian encourages Sai to forge a letter in the Captain's name, relieving Paiyoon of his duties. Assuming the role of mapmaker, Sai witnesses Rian proclaim herself as the ship's Captain after the original Captain falls ill. Over time, Sai grows suspicious of Rian's intentions and discovers that she is deliberately poisoning the Captain and harbors dangerous plans for the Sunderlands.
After realizing the danger Rian poses to the Sunderlands, Sai seeks help from a boy named Grebe. They reached the Sunderlands and were determined to prevent Rian from causing any further harm. In the end, Sai and Bo return to Mangkon safely. Sai portrays the island as barren to prevent the Sunderlands from being exploited by greedy people. Recognizing her self-worth, Sai proudly embraces her identity as a commoner, declining the Lineal of Honor and opting for a cash prize. Under Paiyoon's guidance, she continues her apprenticeship.
