Robert Goulet is "David March", code name "Blue Light". March accepts the assignment as double agent to infiltrate Germany prior to the United States' official involvement in the war. Living in Germany, he aids Nazi activities by broadcasting anti-American propaganda, and inserts essential strategic information in code to his High Commander, the only one who knows he's a double agent. Even March's French paramour, singer Suzanne Duchard (Christine Carere), believes he's on the side of the Nazis. Branded as a traitor and facing danger from many directions, March juggles to keep the suspicious Gestapo from their attempts to entrap him.