During the Cold War, American cryptanalysts spent decades attacking a Soviet cipher system they believed was mathematically unbreakable. This film tracks the Venona Project from its origins in a small Army Signal Intelligence unit through the painstaking work of breaking one-time pad encryption that Moscow had assured its agents could never be read. Historians and intelligence experts explain how repeated use of supposedly single-use key pages gave codebreakers their opening, and how the decrypted cables eventually exposed Soviet spies operating inside the US government and inside the Manhattan Project itself. Archival photographs, declassified cable excerpts, and period footage of Washington and wartime Los Alamos anchor the narration, while interviews lay out the identification of figures tied to atomic espionage. The film follows the project's decades-long secrecy, kept hidden even from most of the government it protected, through to its eventual declassification. What emerges is a case study in how a single cryptographic mistake unraveled a spy network that had operated undetected for years.