Mathilda (Mattie) matures greatly as a result of her experiences during Philadelphia's 1793 Yellow Fever epidemic.

When the story begins, the teenaged (14-year-old) Mattie depends on her mother and grandfather and often does not see eye-to-eye with her mother, who is very hardworking and demanding. She is an ordinary teenager who has not yet had to think seriously about life or death.

Her mother sends her and her grandfather away when the epidemic begins to rage. Mattie then must become self-reliant after her beloved grandfather dies. She doesn't know where her mother is or if she is alive or dead, so Mattie is on her own at a young age. Though Mattie spends a short time shattered and disoriented, she soon pulls herself together, cares for Nell, and rises to adult stature as she reunites with Eliza and the two work side by side as equals.

By the end of the novel, when Mattie has reunited with her mother, she has learned to think for herself and persevere. The tables begin to turn, and her mother, weakened by yellow fever, learns to rely on Mattie. Mattie and Eliza reopen the coffeehouse. Mattie takes on adult responsibilities of making decisions, caring for others, and allowing others to rely on her.

This is a traditional coming-of-age tale in which a young adolescent is forced to grow up very quickly and assume adult responsibilities because of a crisis. Mattie rises admirably to the demands placed on her, behaving sensibly and responsibly.