As the secret countdown to the Normandy invasion gets under way, a fishing boat runs aground on British shores with a hold full of passengers all dead from a mysterious illness. American doctor Frank Brink, who has been working with the British to develop antibiotics in anticipation of a possible Axis biological attack, is summoned to investigate. Interviewing the one surviving member of the crew, a young Frenchwoman who was working with the Resistance, Brink quickly realizes that someone is testing a biological weapon within the French lines. He suspects that it is the pneumonic plague-a horrifying disease with a one-hundred-percent mortality rate.