Adam Dalgliesh, chief superintendent of New Scotland Yard, is the central figure renowned for his dispassionate demeanor and rapid case-solving abilities. A reserved and introspective man, Dalgliesh has also published two books of poetry, reflecting his depth and artistic sensibilities. Despite his insistence on the precedence of criminal investigations over personal privacy, his moral indignation over murder never skews his judicious examination of witnesses and evidence. His confident presence is unshaken even in the unfamiliar environment of John Carpendar Hospital. After suffering a serious head wound in an ambush, Dalgliesh remains undeterred and confronts the nurse he suspects of guilt, despite lacking definitive proof. Ultimately, the resolution of the case leaves him disheartened rather than triumphant, reflecting his complex, introspective nature.

Charles Masterson, Dalgliesh’s handsome subordinate, is depicted as a complacent and ambitious detective sergeant. His resentment towards Dalgliesh's self-discipline and aloof nature underlines his crass ambition. His pursuit of Julia Pardoe, one of the nurses, is driven by calculation rather than genuine affection. Moreover, his cruel treatment of an elderly female witness in London earns Dalgliesh’s reprimand. Nevertheless, Masterson provides capable support to Dalgliesh, whose experience and intuition eventually earn his grudging respect.

Mary Taylor serves as the matron of John Carpendar Hospital’s nurse training school, commanding the same respect and confidence in her domain as Dalgliesh does in his. Her intellect and professional dedication leave a strong impression on Dalgliesh, highlighting her as an intellectual equal. The night of the second murder, she is at a conference, but upon her return, she acts swiftly and efficiently to assist the police investigation, demonstrating her composed and decisive nature.

Sister Ethel Brumfett is the second in command at Nightingale House. This stolid woman, with shrewd eyes and an obsession with the private ward's supervision, surprises Dalgliesh by being the matron’s close friend for twenty years. Her eventual confession to committing double murder leaves several disturbing questions unresolved, emphasizing her complex and secretive character.

Sister Hilda Rolfe, the principal tutor, is characterized by her discontent and abrasive demeanor. Her decision to stay at Nightingale is driven by her unreciprocated love for the student Julia Pardoe, adding a layer of personal conflict and emotional depth to her character.

Sister Mavis Gearing is a clinical instructor and the third senior nurse, noted for her naivety and effusive personality. Regularly entertaining the hospital's married pharmacist, she also displays flirtatious cooperation with Dalgliesh, which hints at her complex interpersonal relationships.

Dr. Stephen Courtney-Briggs, the senior consultant surgeon, exudes arrogance and worldliness. His condescending attitude extends to everyone he encounters, including Dalgliesh, and he openly resents the police inquiry, which highlights his disdain for authority outside his expertise.

Heather Pearce, an unpopular student nurse, meets a tragic end during an intragastric feeding demonstration, where her stomach is burned with carbolic acid. Known as a hypocrite and a potential blackmailer, Pearce was about to exploit her knowledge of one of the Nightingale staff being a former Nazi war criminal, which made her a controversial and threatening figure.

Josephine Fallon stands out as an intelligent loner, older than her peers, who was originally meant to receive the fatal intragastric feeding. An autopsy reveals her death by nicotine poisoning and uncovers her three-month pregnancy, possibly implicating Dr. Courtney-Briggs as the father. Her circumstances add a tragic and mysterious layer to her character.

Madeleine Goodale, a competent and sensible student nurse, draws Dalgliesh’s attention with her brightness. Her close friendship with Fallon led to her inheriting sixteen thousand pounds upon Fallon's death, presenting a potential motive for murder and adding complexity to her seemingly straightforward character.

Julia Pardoe is an attractive student nurse who skillfully navigates the attentions of both Hilda Rolfe and Charles Masterson. Her interactions highlight her allure and the complexity of romantic and professional dynamics within the hospital.

Morag Smith, a somewhat backward maid, feels hounded by the local police but responds positively to Dalgliesh’s courteous approach. Her eventual act of saving him from an assailant showcases her unexpected bravery and contributes significantly to the narrative.