Based on a true story, Sergeant Peter King of the Army Dental Corps, too old to fight, and Private Leslie Cuthbertson, a trainee dental mechanic in the Corps, are thrown together by their passionate desire to see active service. Armed with just two revolvers and a dozen grenades, King persuades Cuthbertson to join him on a mission to occupied France. In a letter to Sir Winston Churchill, King explains their intention to invade France and fight the Germans. After several failed attempts, they finally arrive by boat and stumble across a German radar station. They succeed in blowing up what they believe to be the main Operations Room, but are soon forced to make their escape as the entire compound unexpectedly erupts with gunfire and explosions. After narrow escapes from the Germans and a stray mine in the Channel, the two men are picked up at sea and interrogated as spies. Identified as deserters, they are returned to their barracks to be court-martialled. At the last minute King and Cuthbertson are saved by the arrival of Major Merton, Churchill's Chief Intelligence Advisor. Their letter has been received in Whitehall and Merton corroborates their heroic story. Exonerated of lying, King and Cuthbertson are nevertheless reprimanded by the Court for their behavior, but receive an invitation to tea with the Prime Minister should they ever be in Whitehall.