Klink is made chairman of the Winter War Relief Funds and wants Hogan to make a donation. Hogan refuses, but then they are interrupted by the noise of an approaching squad of Allied bombers. One of the bombers is shot down, and the pilot, one Captain John Mitchell, bails out. Mitchell is captured and brought to Stalag 13. Klink starts interrogating him with his usual efficiency and Mitchell responds with the usual name, rank and serial number. Mitchell slips Hogan a note saying he urgently needs to talk to him about something. But Klink catches this and has Mitchell thrown in the cooler. To Mitchell's fury, there are odd remarks from Hogan that seem to extend his time in the cooler. But once Le Beau lures Schultz out with some cooking, he understands why; it was so Hogan could slip in through one of the tunnels to see him. The cooler is the safest place to talk privately.
Mitchell tells Hogan that there is a top secret Direction Finder (DF) in the plane. The DF is tuned into the signals from two Underground stations, and a cross-fix would reveal their locations. Orders were to destroy the DF if captured, but the crash and destruct mechanism failed to do this. The DF has survived in the front fuselage, which is still relatively intact, but the Germans must not be allowed to get their hands on it. Hogan asks for a sketch of the DF and then he and his men get to work.
There is only one guard posted to the wreckage. So Hogan figures it will be an easy matter to have Carter and Newkirk don German uniform and distract the guard while Le Beau and Kinch remove the DF. But just as Newkirk and Carter approach the guard, the Gestapo show up and take charge of the wreckage. The plan is ruined and they are forced to retreat and rethink.
Over the coffee pot, Major Feldkamp of the Gestapo reveals that the Gestapo has taken charge because the Air Ministry wants to examine the wreckage as it is fairly intact. They will be sending an expert, Dr Wolfgang Becker, for the job. Feldkamp leaves Klink with the file on Becker and orders him to familiarise himself with it. Hogan decides he needs to familiarise himself with it as well. He sends Le Beau and Carter to Klink's office under pretext of cleaning to take photographs of the file. Hogan learns that Becker has one weakness they could exploit - gambling.
So Hogan starts converting the barracks into a casino to raise funds for Klink's Winter Relief Fund. Klink agrees to this as he is desperate to raise money for the funds. Hogan also has Carter make a replica of the DF using Mitchell's sketch. Meanwhile, Becker arrives and Hogan and Co observe him finding the DF.
The casino is soon in full swing, and even Klink is on a winning streak. But then Hogan receives bad news - Becker is leaving for Berlin that night with the DF. Worse, he has the DF in a briefcase that is attached to his wrist. He orders Carter to bring in the fake DF and a few smoke bombs, and Newkirk to be ready to open the briefcase. Hogan goes to see Becker and invite him to the casino, saying the invitation came from Klink. Although he has a deadline to meet, Becker cannot resist and agrees to a brief fling in the casino before leaving.
Once Becker enters the casino, Hogan lights the smoke bombs to simulate a fire. Under cover of the smoke, Kinch knocks Becker out while Newkirk opens the briefcase and makes the switch. When Becker recovers, he assumes he passed out from the smoke. He is soon on his way with the fake DF.
Klink makes a nice tidy sum for the Winter Relief Fund. He wants to deduct some of it as expenses, but Hogan hints that he would be wiser to send it all to Berlin or it could be the Russian Front. Klink takes Hogan's advice.