Hogan and Co are trying to blow up a bridge when an SS car approaches, with SS Captain Steiner and two others (Professor Bauer and Dr Riemann). Carter and Hogan, who are dressed in German uniform, try to stall by demanding papers. But Steiner seems to see through the deception when he stamps on Carter's foot and the pain causes him to lapse into English. Steiner takes Hogan and Carter into the woods at gunpoint - but then just leaves them there while he departs in his car. The incident has Hogan ordering his men to abort the job.
Next morning, Hogan fears the SS will come after them and orders Klink's calls to be monitored. But he cannot understand Steiner's conduct and also suspects Steiner did not have papers. Then Steiner, Bauer and Riemann arrive in camp. Believing the game is up, Hogan orders his men to start evacuating. But Steiner says nothing to Klink about the incident; instead, the scientists say they are on their way to a meeting in Paris and ask Klink to put them up for a few days. Klink summons Hogan and tells him to have Le Beau prepare his French cooking. Hogan is astonished and relieved. Things return to normal in the barracks.
Later in the barracks, Steiner, Bauer and Riemann meet up with Hogan and Co and explain. The scientists have been working on atomic research. The atomic bomb is getting so close to reality that the scientists refuse to work for Hitler. They were actually on their way to the Swiss border when they met up with Hogan before. They have come to Stalag 13 because it was the most likely place for people to speak English. Now they want Hogan's help in getting them to Switzerland, but Hogan offers England instead if they can prove their credentials. Their credentials are five flasks of heavy water. Then Baker, who is monitoring Klink's phone, gets further proof of credentials - Major Hochstetter is on his way to arrest the men.
Hochstetter arrives to find the men (now disguised as prisoners) have disappeared and starts a search. He fails to find them, but suspects they are nearby because their car is still in the camp. He says he will be at Stalag 13 every single day until he does find them. Faced with this and the extra guards, Hogan sees only one way to smuggle the men out - Hochstetter's car. But the trunk can only hold one person, so Hochstetter's car has to make three trips. So for the next three days they contrive to smuggle out one man at a time in Hochstetter's car, with the Underground ready to receive them in Hammelburg. The scientists make it out safely, but there is still Steiner and the heavy water. So Hogan makes it look as if the men hid in the cooler for the benefit of Klink and Hochstetter. While Hochstetter investigates, Steiner and the heavy water are hidden in his car. Hochstetter thinks the men are still in the camp and orders Stalag 13 to be put under Gestapo control until he finds them. Klink is annoyed and vows his own revenge on the men.
Later, Klink finds out the reward for the men is 50,000 marks. He and Schultz believe they have entitlements to the reward and start quarrelling over how to divide it. Hogan leaves them to it.