- Edward George Armstrong - a Harley Street doctor
- William Henry Blore - a former police inspector, now a private investigator
- Emily Caroline Brent - an elderly, pious spinster
- Vera Elizabeth Claythorne - a young woman on leave from her position as a sports mistress at a girls' school
- Philip Lombard - a soldier of fortune
- General John Gordon MacArthur - a retired World War I general
- Anthony James Marston (Tony Marston) - an amoral and irresponsible young man
- Ethel Rogers - the cook/housekeeper and Thomas Rogers' wife
- Thomas Rogers - the butler and Ethel Rogers' husband
- Lawrence John Wargrave (Mr Justice Wargrave) - a retired criminal judge
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