Bob Sharkey (Cagney) is an instructor of the 77th group of espionage agent candidates to be trained in the United States to infiltrate Nazism-occupied Europe. He is alerted that one of the students is a German agent and told to identify him or her. Sharkey is able to determine that the enemy agent is "Bill O'Connell" (Conte), who performs too well when he succeeds in a field problem designed to cause the novices to make mistakes. Sharkey's boss, Charles Gibson (Walter Abel), confirms that O'Connell is really Wilhelm Kuncel, one of Germany's top agents, but tells Sharkey to pass him, as they know Kuncel's mission is to determine the date and location of the Operation Overlord. They intend to provide Kuncel with false information to pass along to his superiors.