Henry is a 48-year-old neurosurgeon who lives a comfortable upper-class life in Fitzrovia, London. His days are heavily structured by his demanding medical career, but he finds deep satisfaction in his routines and domestic life. As he moves through his Saturday errands and leisure activities, his highly rational, scientific mind constantly filters the world around him.

Baxter is an agitated, volatile man who becomes involved in a street altercation with Henry on the barricaded roads of London. He displays physical and behavioral signs of Huntington's Disease, a genetic condition that causes uncontrollable mood swings. Viewing the affluent Henry as a symbol of the societal privilege he lacks, Baxter fixates on him following their dispute over vehicle repairs.

Daisy is Henry and Rosalind's daughter, an independent young woman living abroad. She recently won the Newdigate Prize and has a soon-to-be-published book of poetry. She possesses a highly intuitive perspective that frequently clashes with her father's pragmatic worldview, especially regarding global politics and military intervention.

Rosalind is a successful lawyer working for a newspaper. She met Henry years ago when he assisted in a surgery to remove a tumor causing her blindness. She brings balance to Henry's life by teasing him about his habits while providing a strong, supportive presence in the household.

Theo is Henry and Rosalind's teenage son. He is an aspiring blues guitarist who leads a relaxed lifestyle characterized by sleeping late and rehearsing with his band. His musical talent provides his highly logical father with a rare connection to human emotion and artistic transcendence.

John is Rosalind's father, a renowned poet who lives in a French chateau. He played a significant role in fostering his grandchildren's artistic talents. He maintains a polite but distant relationship with Henry, as the two men operate in vastly different professional spheres.

Jay is an American medical colleague and frequent squash partner of Henry's. He is a passionate defender of the impending war in Iraq, displaying a political certainty that causes Henry to question his own views. He shares Henry's competitive drive on the court.

Lily is Henry's mother, a former competitive swimmer for Middlesex who now resides in a care facility. She suffers from advanced dementia, leaving her disoriented and living in fractured memories. Her deteriorating condition weighs heavily on Henry.

Taleb is an Iraqi professor of ancient history who acquainted Henry with the horrors of imprisonment under Saddam Hussein. His firsthand accounts heavily shape Henry's pragmatic views on international military intervention.

Nark is one of Baxter's companions who accompanies him during the confrontation with Henry. He follows Baxter's orders and provides physical intimidation during the street encounter.

Nigel is Baxter's second companion during the traffic incident. He stands by to assist Baxter in threatening Henry for repair money on the barricaded street.

Andrea is a hospital patient who successfully undergoes a procedure performed by Henry. She enthusiastically journals about her experience and expresses a keen desire to become a neurosurgeon herself.