After disposing of the final traitor, Smith rejoins Schaffer and the others and they enact their daring plan to escape from Germany after escaping the Schloss Adler at last.
Commandeering a bus with a snowplow that was under repairs, the group shoots their way through several luckless Nazi soldiers, they hightail it to a German airfield, where they're to rendezvous with Colonel Turner, who planned the whole mission along with Vice-Admiral Rolland. Several more crashes, explosions and shootouts later, and everyone makes it safely aboard the waiting plane (a captured German cargo plane), and pilot Carpenter takes off.
En route back to England, Turner, carrying a Sten sub machine gun given to him by Admiral Rolland, congratulates Smith and Schaffer on a job well done, and asks to see the books containing the names of Nazi spies in England, which Smith still has on his person. Smith refuses, dropping the bombshell that Turner already knows all the names in them, because he himself is a Nazi spy, and his name is in them! Turner scowls and aims his Sten at Smith, only to discover it won't fire.
Smith reveals he and Admiral Rolland were suspicious of the Colonel even before the mission began, and before leaving Smith removed the firing pin from a Sten - which Rolland then gave to Turner when he left. Unarmed and facing personal disgrace and imprisonment back in England, to say nothing of the very strong possibility of a firing squad, something Smith coldly reminds the Colonel he'll probably be facing for his treason, Turner calmly sets the useless gun down, rises, and opens the door of the plane while they're still in flight.
Nobody moves to stop him.
He pauses and looks back over his shoulder at the still seated commandos, and we cut to a reaction shot of them, and then Smith gets up and goes to still open door, only now there's no sign of Turner: he jumped out to his death offscreen. Smith shuts the door.