Fiona is a 59-year-old High Court judge in London's Family Division. She lives a comfortable but increasingly isolated life in the Gray's Inn neighborhood. Over the years, she has prioritized her judicial career over having children or maintaining her marriage, bringing strict legal order to family disputes in the courtroom while her own domestic life frays. She is highly focused on an urgent medical case involving a teenager refusing treatment.

Adam is an intelligent, artistic 17-year-old raised strictly within the Jehovah's Witness faith. He suffers from a severe case of leukemia and refuses a life-saving blood transfusion because his religious teachings consider it unclean. He writes poetry and plays the violin, displaying a sensitive and curious spirit despite his severe illness.

Jack is Fiona's 59-year-old husband. Feeling alienated by his wife's chronic workaholism and their dormant sex life, he proposes opening their marriage so he can have an affair. He generally displays a loving and kind disposition, making his sudden demand for extramarital intimacy particularly jarring to Fiona.

Marina is the social worker assigned to Adam Henry's case. She works within the hospital system to advocate for Adam's welfare and monitors his physical and emotional status during his hospitalization.

Kevin is Adam's father and a devout member of the Jehovah's Witness community. He strictly follows his church's biblical interpretations prohibiting blood transfusions, believing that obedience to God supersedes Western medical interventions.

Naomi is Adam's mother. Like her husband, she adheres strictly to Jehovah's Witness teachings and refuses to authorize a blood transfusion for her son.

Nigel is Fiona's clerk at the Royal Courts of Justice. He manages her schedule, organizes her files, and acts as a gatekeeper for her urgent legal matters.

Mark is Fiona's legal colleague and friend. Beyond their professional crossover at the Royal Courts of Justice, he plays music with Fiona, partnering with her for the annual Christmas concert.

Melanie is a 28-year-old statistician. Jack identifies her as the woman he wishes to have an affair with to satisfy his physical needs outside of his marriage.

Leslie Grieve is the legal counsel representing Kevin and Naomi Henry. He argues that Adam's refusal of the blood transfusion is an independent choice made by an intelligent young man.