In Hunters in the Stream, Riley Fitzhugh goes through officer training and is assigned to PC 475, a new anti-U-boat vessel stationed in Key West. The 475 is nicknamed Nameless by her crew because patrol craft vessels were only given numbers.

Nameless cruises the Gulf of Mexico in search of U-boats, goes to the rescue of a sinking oil tanker, stops in Havana for meetings with the Cuban Navy, and learns of a possible secret German U-boat fueling station in the wilds of eastern Cuba. Nameless locates the base and destroys it with the ship's gunfire and a coordinated small-arms attack led by Fitzhugh and his shore party. Later, another U-boat is reported damaged and sinking. The German survivors capture a Bahamian turtle boat, murder the crew, and head for Cuba, thinking that the fuel dump is still in operation. Fitzhugh and the Nameless pursue through the tangle of mangroves and Cuban keys, find the Germans, and finish them off in a shootout. Along the way, Fitzhugh meets Ernest Hemingway and toward the end tells him about the Nameless's adventures. Hemingway thinks about adapting the story for his own. Fitzhugh and Hemingway's wife, writer Martha Gellhorn, also meet and feel some mutual stirrings--and give in to them.