Olaf Christiansen was a British Intelligence department head. He was secretly a German spy, however. After Major John Smith and Vice Admiral Rolland pinned the infiltration of MI6 on Christiansen's control of his department, sending out classified information to the Nazis and bringing in useless, out of date information, Rolland concocted a daring plan to expose Christiansen and his fellow spies Lee Thomas and Edward Carraciola. The three along with George Harrod and Torrance-Smythe were selected to accompany Smith and American officer Morris Schaffer to rescue what they were told was one General Carnaby from the Schloss Adler in Bavaria.
During the mission, Christiansen and his fellow traitors did all they could to sabotage the mission, which they assumed was genuine. They murdered Torrance-Smythe and Harrod. When the group was captured in the Zum Wilden Hirsch by SS troops commanded by Colonel Weissner, Weissner took Smith and Schaffer with him in his car, but sent Christiansen and the other two to the Schloss Adler for questioning. Once there, he, Thomas and Carraciola immediately revealed themselves to Colonel Kramer as spies, and were welcomed at the dinner table in the Golden Hall, where General Carnaby was being interrogated.
But suddenly Smith and Schaffer appeared. Smith, pretending to be SS agent Johann Schmidt, revealed Carnaby as actor Cartwright Jones, and managed to convinced Kramer and Reichsmarschal Rosemeyer that Christiansen and his two fellow traitors were also imposters. He insisted the real Christiansen, Carraciola and Thomas had been captured in Britain, and the three men present were Allied double-agents. Desperate to prove himself as a loyal Nazi, Christiansen, prompted by Schmidt, wrote down the names of all his contacts in Great Britain - including his immediate superior, Colonel Wyatt-Turner. Thomas and Carraciola did likewise. Only then did "Schmidt" reveal it was a lie all along; he really was John Smith.
Kramer, Rosemeyer and the other Schloss Adler personnel present were knocked out with injections of sleeping drugs, whilst Christiansen and his two fellow spies were taken as hostages. Once the group got to the castle's cable car control room, Christiansen and his companions broke free and took Schaffer hostage, demanding their release in exchange for the American's life. Smith obliged, and, releasing Schaffer, Christiansen and his comrades boarded the cable car and tried to get away. Smith however leaped onto the roof of the descending cable car, intent on stopping them.
After Smith killed Carraciola by knocking him off the roof when he climbed up to shoot him, Christiansen tried his luck, and was stabbed in the hand. Thomas managed to pull him back inside. Smith planted a bomb, then jumped onto a passing cable car as it ascended. When the bomb blew, it rocked the cable car, and Christiansen slid out. He tried to grab the door with his injured hand, failed, and fell to his death.
- Christiansen's first name suggests he is of German descent, although this is never confirmed.
- In the film, Christiansen fights Smith on top of the moving cable car, but slips and grabs his opponent's legs. It is Thomas whose arm (not hand) is injured by Smith. Despite his pleas, Christiansen is shaken loose by Smith and falls to his death well before the bomb destroys the cable car.
- Christiansen has a British Army captain's rank insignia in the film. His rank isn't given in the novel.
- Some sources for the film identify him as "James Christiansen."