Lieutenant Morris Schaffer was an American military officer attached to the Office of Strategic Services during World War II. Schaffer was a very practical-minded man whose quiet nature could easily be mistaken for stupidity. In truth, he was quite intelligent, and highly skilled at sabotage, counterintelligence operations and stealth.
When General George Carnaby's plane crashed in Bavaria near Garmisch-Partenkirchen, head of MI6 Vice-Admiral Rolland concocted a daring plan to rescue him before the Nazis could force Carnaby to tell what he knew of the Allied plans for the second front, Schaffer was assigned to be second in command of the rescue team, serving under Welsh officer Major John Smith. He was unaware that "Carnaby" was in reality actor Cartwright Jones.
At the initial start of the mission, Schaffer believed they really were attempting to rescue an American general, and knew nothing of Rolland's plan to root out Nazi spies using the Carnaby rescue as a cover. Following the deaths of team members George Harrod and Torrance-Smythe, however, Smith let Schaffer in on their true mission. Smith had requested an American because with MI6 so deeply penetrated by the Nazis, he worried he couldn't trust a British officer.
Disguised as a German lieutenant, Schaffer accompanied Smith to the Zum Wilden Hirsch, where he met Ethel, an Allied agent who'd been posing as a waitress there named Heidi. They two fell in love. They were temporarily captured by the SS. Olaf Christiansen, Lee Thomas and Edward Carraciola were taken away whilst Schaffer and Smith were driven to the nearby SS barracks in a Mercedes. They escaped when Smith convinced the officer in charge, Colonel Weissner, that he was Bernd Himmler, Heinrich Himmler's son. The two stole Weissner's Mercedes and pushed it into the river, fooling Weissner and his men into thinking they'd crashed and drowned.
They gained entrance to the Schloss Adler by riding on the roof of the cable car carrying Christiansen, Thomas and Carraciola, who were actually Nazi spies, and were helped inside by Mary Ellison. Schaffer used his abilities at sabotage to cripple a German helicopter to prevent its use in transporting Cartwright Jones to Berlin. After he and Smith knocked out and tied up the pilot, Schaffer dressed in the pilot's coveralls and pretended to work on the helicopter's engine, but in reality, he was sabotaging it. The two then journeyed to the Golden Hall, where Jones, as Carnaby, was being interrogated by Colonel Paul Kramer and Reichsmarschal Julius Rosemeyer.
No one was more surprised than Schaffer when Smith suddenly revealed himself as Captain Johann Schmidt of the SS, and forced the American to drop his gun. Over the next several minutes, a furious Schaffer watched as Schmidt pursued Kramer and Rosemeyer of his true Nazi loyalties, and that Christiansen, Thomas and Carraciola were impostors. But after the three, in an effort to prove themselves, wrote down the names of all their contacts in Britain, Schmidt reverted back to Smith, revealing he'd tricked Kramer and Rosemeyer to get the names of the other British spies working for the Nazis.
Despite an intrusion by Captain von Brauchitsch of the Gestapo, Schaffer, Smith, Mary and Jones were able to knock all of the Nazis out and escape with the lists of names, taking Christiansen, Thomas and Carraciola with them as hostages. They attacked Schaffer and tried to escape on one of the cable cars, but were stopped and killed by Smith. Schaffer, recovering from his injuries, joined Smith, Mary and Jones in fleeing the Schloss Adler in the remaining cable car. They returned to the nearby village where Schaffer was reunited with Ethal and stole a convenient bus, driving it to Oberhausen Airfield, where they were picked up in a plane by Colonel Wyatt-Turner and Wing Commander Carpenter.
While returning to England, it was discovered that Wyatt-Turner was himself a Nazi spy. He committed suicide by leaping from the plane. Schaffer afterwards promised to take Ethel to the Savoy Grill and treat her to a plate of Aberdeen Angus.
In the film, Schaffer is quieter, and much more ruthless, killing numerous Germans during the mission. He also isn't very mechanically inclined, and he sabotages the helicopter by simply murdering its pilot. There also isn't a romantic subplot between him and Heidi.