The (O)ffice of (S)trategic (S)ervices' Cmdr. Brady (Patric Knowles) forms Operation "Applejack" (based on a composite of actual incidents during WWII) and sends Lt. (j.g.) Philip Masson, U.S.N.R. aka John Martin as spy Philippe Martine (Alan Ladd) along with Miss Ellen Rogers posing as her college roommate, Madame Elaine Duprez (Geraldine Fitzgerald) and Robert Bouchet, Tech Sgt., A.U.S. as Albert Bernardito (Richard Benedict) to acquire secret Nazi plans. After nearly getting caught they succeed and get new identities. However they discover a secret that could change the war and risk their lives to get the information back to London before it jeopardizes their lives. Martine and Duprez then get reassigned to assist fellow spy Frank Schmidt aka Parker (Richard Webb) embedded in the German army so he can relay important Nazi troop movements. Then she is discovered by an old foe, Col. Paul Meister (John Hoyt) but Martine must radio the vital information and misses her.