In Diamonds at War, the narrative unfolds with Mr. Eustace Leffingwell, a confident diamond merchant, who understands the critical role that industrial diamonds play in the war effort. As the story begins, three passengers arrive in New York City from Lisbon, each with their own reasons for seeking Leffingwell. Among them is the charming Miss Patricia Moon, who is carrying a briefcase that holds more than just personal items; it contains diamonds that could be vital to the war effort. Accompanying her is Alexander Barker, a young American who has been administering Quaker relief in Spain, and Sir Roger Payne Wexford, a British official. The stage is set for a thrilling encounter filled with intrigue and danger.