In the sixteenth episode of "Kusuriya no Hitorigoto," a mysterious case is investigated involving the will of a deceased metalworker, which Maomao presents to Jinshi from Lakan. The will divides the metalworker's assets among his three apprentice sons in an unusual manner: the eldest son inherits the workshop, the middle son receives furniture, and the youngest is given a fishbowl, along with an invitation for the brothers to reminisce about the past during a tea party.

Maomao, fascinated by the strange bequests, disguises herself as Basen's assistant and enters the workshop. There, she discovers a conspicuously placed metal piece and a chest secured with a lock that doesn't match the key. The chest is positioned near a distinctive high window in Western style, obscured by a chestnut tree.

The plot revolves around the tea gathering stipulated in the will. While the youngest son is excited to understand his father's last wishes, his older siblings are skeptical. The turning point comes when Maomao observes sunlight passing through the unique window and realizes it aligns with the base of the glass fishbowl, which fits perfectly into a cabinet beneath the window. She fills the bowl with water and discovers a mechanism where the sunlight, focused through the bowl, melts a solder blocking the keyhole, allowing the middle son's key to work. This reveals a series of drawers, each containing a different metal. Maomao deduces that these metals might be the recipe for a special alloy.

This discovery initially discourages the older brothers, who dismiss it as their father's final joke and prepare to leave. However, the youngest brother's plea for unity and mutual respect reflects the father's true intentions, highlighting the unique strengths each brother brings to the shared project.

Maomao's final advice to the youngest son—to seek treatment from an apothecary named Luomen in case of emergency—concludes her involvement in the case, which is now resolved to Lakan's satisfaction. The episode showcases the brothers' newfound collaboration, with their roles in the business redefined to leverage each sibling's strengths.

The episode ends with a conversation between Jinshi and Lakan, touching on themes of talent recognition and the value of courtesans, drawing a subtle parallel to the story of the metalworker's sons, with broader implications for talent and worth in society.