Cartwright Jones was an American actor. When Vice Admiral Rolland of the British military concocted a plan to root out German spies, Jones was enlisted to impersonate an American general named George Carnaby. Carnaby held important information regarding the second front, but the Germans didn't know what he looked like. The cover story to trick the spies, especially Colonel Wyatt-Turner, was to have Jones get shot down over Bavaria and be taken to the Schloss Adler, where he would be held by the Nazis, prompting a perfect excuse to assemble a commando team comprised of both confirmed and suspected traitors to go and rescue him, whereupon Major John Smith would expose them and obtain from them the names of all their fellow spies in Britain.
Jones, as Carnaby, was treated relatively well by his captors, treated to a lavish dinner in the Schloss Adler's Golden Hall. Reichsmarschal Julius Rosemeyer held off the Gestapo and their torture techniques, hoping to learn the information about the second front in a civilized and painless manner, but "Carnaby" stubbornly refused to give him and Colonel Kramer anything more than his name, rank and serial number. Kramer brought in Edward Carraciola, Olaf Christiansen and Lee Thomas, three of the commandos sent to rescue Carnaby, and revealed them as Nazi spies in an effort to demoralize him. Still Jones didn't break. Not only did he already know there would be traitors in the rescue team, he of course had no actual information about the second front because he wasn't actually Carnaby.
When he refused to talk, Rosemeyer finally relented and prepared to let Kramer have "Carnaby" tortured, when John Smith appeared with American lieutenant Morris Schaffer. As part of his plan to sow distrust among the Germans, and the three traitors in particular, Smith, posing as "Johann Schmidt," forced Jones at gunpoint to give his real name to Kramer and Rosemeyer. After Smith got the list of names he needed and reverted to his original, true loyalties, thereby incapacitating Kramer and the others, Jones went with him during his group's escape from the Schloss Adler, helping keep an eye on the captured Carraciola, Christiansen and Thomas. Having fulfilled his mission for Admiral Rolland, Jones successfully escaped with Smith, Schaffer, Mary Ellison and Ethel to Wing Commander Carpenter's waiting Lancaster, and returned to Britain.
In the film, Jones is a corporal in the US Army in addition to being an actor.