John Smith was a Welsh major in the British military during World War II. He received his training in the British Life Guard, Black Watch, and before the war had studied at the University of Heidelberg in Germany. He was bilingual, and spoke both English and German. During the war, he was employed by MI6 was a secret agent. Under the alias Johann Schmidt, he pretended to be of German birth, and spent several years undercover as an SS officer where he befriended Colonel Wilhelm Wilner. In this capacity, he fed Wilner useless and out of date information.
When MI6 became plagued with German spies, Smith was recalled to Britain where he was assigned by Vice Admiral Rolland and Colonel Wyatt-Turner to a dangerous mission to discover the identities of the spies. He was chosen because Wyatt-Turner considered him the best agent in the British Secret Service. An American actor named Cartwright Jones, impersonating General George Carnaby, had allowed himself to be captured and held at the Schloss Adler in Bavaria.
Smith was to lead a team to rescue "Carnaby." Among the men were three of the suspected spies, Olaf Christiansen, Lee Thomas and Edward Carraciola. Smith's plan was to use the mission as a pretense for learning the names of the three's contacts in Britain. He was partnered with American lieutenant Morris Schaffer, but didn't tell Schaffer about Jones because he didn't trust him. Smith's girlfriend and longtime collaborator Mary Ellison was also selected for the mission.
Smith, Schaffer and their team disguised themselves as German soldiers and parachuted in near the Schloss Adler. During the jump, radioman George Harrod was killed under mysterious circumstances. Later, when the group visited the Zum Wilden Hirsch, Torrance-Smythe disappeared and Smith found him murdered. After this, with no more loyal allies left, Smith was forced to rely on Morris Schaffer, and told him the truth about "General Carnaby." Arrested by SS troops led by Colonel Weissner, Smith and Schaffer's quick thinking got them out of that jam, but separated them from Christiansen, Thomas and Carraciola.
Together, Smith and Schaffer infiltrated the Schloss Adler via its cable car system, riding on one of the cars which was taking Christiansen, Thomas and Carraciola up. Once inside the castle, the two were assisted by Mary and fellow agent Ethel. Finding Jones being questioned by several high-ranking Nazi officers including General Julius Rosemeyer and Colonel Paul Kramer in the Golden Hall, Smith pretended to betray Schaffer in order to persuade Christiansen, Thomas and Carraciola to write down the names of their contacts in Britain. He "proved" who he was by showing Kramer some scars on his arm, and having them phone Wilhelm Wilner.
The intrusion of Gestapo officer Captain von Brauchitsch nearly ruined things, but Mary entered and held von Brauchitsch at gunpoint. He and Schaffer then injected Rosemeyer, Kramer and the other Nazis present with nembutal and took Christiansen, Thomas and Carraciola as hostages, and staged a daring escape from the castle.
Smith's efforts to contact Rolland and Wyatt-Turner by radio to arrange a rescue were stymied by a quick-thinking SS officer who shut off the power, forcing the British to go to Plan B, a dangerous pickup by plane at the nearby Oberhausen Airfield. Despite his ruthless and calculating nature, Smith at one point saved the life of a German radio operator amidst the chaos as he and the others escaped the castle.
Smith, Schaffer, Mary, Jones and their three unwilling captives made their way out, but once at the cable cars, the captive spies made their move, taking Schaffer hostage and threatening to kill him if Smith didn't let them go. Smith complied, and they released the American and boarded a cable car, but Smith jumped on top of it. He fought them, knocking Christiansen off and to his death, and then, seeing a second, empty cable car coming, planted a bomb on the roof of the on carrying Thomas and Carraciola, then jumped off. Thomas and Carraciola's cable car exploded, killing them. Returning to his friends, Smith and the others took the second car down to the village, where Ethel awaited them with a bus.
Using the bus, the five of them drove to Oberhausen, where Colonel Wyatt-Turner arrived aboard Carpenter's Lancaster. They escaped in it. When Wyatt-Turner asked for the list of spies, Smith refused, because the Colonel's name was in it! In fact, Smith had suspected him for some time, and so as a precaution had the firing pin on Wyatt-Turner's gun filed down. Faced with being executed as a traitor, Wyatt-Turner leaped from the plane to his death. Smith didn't try to stop him. He returned to Britain with the names he'd gotten, ridding MI6 of its spy infestation, and also ended up marrying Mary Ellison.
- Smith in the film is essentially identical to his novel counterpart, however he is depicted as being far more ruthless when it comes to his work, murdering numerous Germans in cold blood in situations outside of direct combat. For example, although he rescues a radio operator in the novel, he kills this same character in the film.