Phineas is a 26-year-old railroad foreman recognized for his physical strength and job competence. While packing explosive powder with a tamping iron, he suffers a severe accident when the iron shoots upward through his face and frontal lobe. His physical survival and the alterations in his personality initiate extensive medical discussions regarding how the human brain functions.

Dr. Harlow is a 19th-century physician practicing in Vermont who directs Gage's medical recovery. Working before the scientific community understands germ theory, he cleans the wound and monitors his patient closely. He documents Gage's physical healing as well as the noticeable alterations in his social behavior and judgment.

Dr. Bigelow is a Boston surgeon who brings Gage to a larger medical audience. As a proponent of the idea that the brain operates as a unified system, he uses Gage's survival to support his theories. He relies on physical evidence and eyewitness accounts to convince skeptical doctors that the injury actually occurred.

Paul Broca is a medical researcher whose work addresses specific questions regarding brain localization. He identifies an anatomical section in the left frontal lobe responsible for producing spoken words. His discoveries help move the scientific community toward exact brain mapping instead of generalized theories.

Carl Wernicke is a scientist exploring regional specialization within the human brain. He locates a specific brain section connected to language comprehension. His findings support the theory that discrete brain regions control highly specialized functions.

Antonio Damasio is a modern neuroscientist examining patients with frontal-lobe injuries. He observes that his contemporary patients share behavioral traits with Gage, particularly the retention of logical abilities combined with poor social decision-making. His clinical work connects historical case files to current neurological understanding.

Hanna Damasio is a modern neuroscientist who applies advanced computer imaging to historical medical cases. She utilizes digital models to map the precise path the tamping iron took through Gage's skull. Her technological approach provides an anatomical explanation for the behavioral changes noted by 19th-century doctors.

Gage's mother provides primary accounts of her son's daily routines following his recovery. She maintains contact with medical professionals and observes his general physical health. Her correspondence offers insight into his ongoing abilities and routines outside of a clinical setting.

Dr. Nelson Sizer is a phrenologist who analyzes the reports of Gage's altered behavior. He uses the case to argue that specific areas of the brain control discrete feelings and social functions. His interpretations represent the ongoing debate over brain localization during the 19th century.