In January 1917, a German telegram crosses British telegraph lines, intercepted by the codebreakers of Room 40. The message, sent by Foreign Secretary Arthur Zimmermann to Mexico, proposes a wartime alliance against the United States, offering Mexico back Texas, New Mexico, and Arizona if America enters the war on the Allied side. This film follows the small, unconventional team of cryptographers racing to break the cipher before Britain's options run out, then facing a harder problem than the code itself: how to reveal what they have found without exposing that Britain has been reading American and neutral cables for years. Historians walk through the diplomatic gamble that followed, weighing the telegram's potential to drag a reluctant United States into the war against the risk of wrecking trust between the two nations if the deception behind the intercept ever came out. Archival documents, period photographs, and expert interviews carry the account of one intercepted message that helped reshape the war's outcome.