In post-apocalyptic New York, three tribes of survivors (the Norms, the Mutates and the Upriver People) vie for the right to exist. When the treacherous Upriver People attack the Norms, kill their chief and take their people captive, two Norm refugee men must find a way to ally with the Mutates, who have previously kidnapped Norm women in an effort to reproduce healthy children, to rout the Uprivers, who also seek to kill off the Mutates.

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.