Nora is a twenty-year-old orphan raised by Dr. Horace Croft after surviving a cholera outbreak as a child. She serves as Croft's secret clinic assistant, skillfully dissecting corpses, conducting experiments, and producing medical illustrations. She resents societal expectations that relegate women to the domestic sphere and fiercely guards her illegal medical practice from outsiders.

Croft is a brilliant, absent-minded surgeon at St. Bartholomew's Hospital who operates a private clinic from his London home. He studies plant and animal anatomy in addition to human medicine. His insatiable curiosity about biology often supersedes conventional etiquette. He values efficiency and results over societal norms, maintaining a chaotic but productive household.

Daniel is a young, fastidious doctor from an affluent family who comes to study under Dr. Croft. He values traditional Victorian social standards and believes men and women should inhabit separate spheres. Despite his rigid adherence to propriety, he struggles to mask his intense emotional sensitivity to suffering patients.

Dr. Vickery is the chief of surgery at St. Bartholomew's Hospital. He strictly adheres to traditional medical practices and actively resists innovative treatments. He holds a deep resentment toward Dr. Croft and frequently uses his authority to demand compliance from younger doctors.

Mrs. Phipps is Dr. Croft's stalwart housekeeper and the only domestic employee willing to tolerate the morbid realities of his home clinic. She runs the household with a tough exterior but possesses a tender, maternal affection for Nora. She provides practical grounding for the eccentric medical professionals living under her roof.

Harry is a surgeon at St. Bartholomew's Hospital who recently spent two years serving in the navy. Coming from a less affluent background than his friend Daniel, he maintains a relaxed attitude toward societal expectations. He approaches medical innovation with an open mind and readily engages with unconventional ideas.

Salvio Perra is a professor of anatomy visiting from the University of Bologna in Italy. Coming from a medical community that historically accepts female practitioners, he offers an outside perspective on the rigid social conventions governing London's medical establishment.

Mae is Daniel's fiancée, a refined woman who perfectly embodies the delicate, conventional ideals of Victorian femininity. She expects her future husband to behave like a proper gentleman and prefers not to hear the grim details of his medical profession.

Mrs. Gibson is Daniel's affluent mother. She values social propriety above all else and heavily disapproves of her son's decision to pursue a hands-on surgical career rather than establishing a respectable private practice near their home.

John Prescott is a powerfully built friend of Dr. Harry Trimble who suffers from a severe hernia. His sudden medical emergency brings him to Dr. Croft's clinic, forcing the resident doctors to make difficult decisions regarding his treatment.