Jessie is a thirteen-year-old girl who spends her days completing chores and attending the village school in Clifton, Indiana. She possesses a bold streak that pushes her to explore the boundaries of her environment. When a diphtheria outbreak threatens the town's children, she accepts a dangerous assignment to leave her home and find medical help.

Ma serves as the village midwife and nurse in Clifton. When the local doctor stops providing effective pills, she takes over caring for the sick residents using her own medical knowledge. Recognizing the severity of the recent diphtheria outbreak, she recruits her daughter for a risky mission to find outside help.

Pa works as the village blacksmith in Clifton, hammering iron and shaping horseshoes. He deeply immerses himself in the 1840s frontier lifestyle and insists on maintaining old-fashioned customs. His devotion to this traditional routine often leaves him disconnected from the more urgent problems facing his family.

Miles Clifton is a wealthy investor who founded the historical preserve where the Keyser family resides. He maintains strict control over the environment and funds various scientific research projects. His influence dictates the rigid rules that govern the residents' daily lives.

Frank Lyle is a researcher who works on long-term medical projects. He receives funding from wealthy investors to study human health and disease progression. He views his scientific goals as vastly more important than the well-being of individual people.

Hannah is Jessie's older sister. She conforms closely to the expectations of a young woman in the village, focusing her attention on domestic duties and social standing. She prefers safety over adventure and avoids breaking community rules.

Bob works as a reporter for the Indianapolis Gazette. He investigates unusual local claims and insists on gathering verifiable facts before writing his newspaper stories. He maintains a professional demeanor but shows genuine concern for young sources who approach him for help.

Katie is the youngest member of the Keyser family. She contracts diphtheria early in the story, making her survival a primary motivation for her family's subsequent actions.

Andrew is one of Jessie's younger brothers. He shares a strong family bond with Jessie and enjoys a playful relationship with her inside their crowded cabin.

Nathan is a younger brother in the large Keyser family. He spends his days in the village, surrounded by his many siblings.

Bartholomew is another younger brother in the Keyser family. He resides in the small frontier home and participates in the daily routines of the village.

Dr. Fister acts as the medical doctor for Clifton. He previously distributed highly effective medicine but now relies entirely on basic folk remedies to treat his patients, causing concern among the villagers.

Mr. Smythe teaches the village children in the local schoolhouse. He enforces strict classroom discipline and expects students to memorize historical facts on command.

Mrs. Spurning leads student field trips while wearing period-appropriate clothing. She delivers prepared historical narratives to visitors, offering an official version of the site's history.

Nicole is a student participating in a school field trip. She wears modern clothing and represents the contemporary world that exists beyond the boundaries of the isolated frontier village.

Mrs. Tyndale rides the public bus in Indianapolis. She freely strikes up conversations with fellow passengers and provides practical directions to out-of-town visitors who need help finding their way around the city.

Mr. Seward runs the general store in the village. He closely follows the rules established by the town's founders and expects strict obedience from his family members.

Chester is a young resident of Clifton who helps around the village. He harbors strong feelings for one of the older Keyser daughters and pays close attention to her well-being.