Hogan is out on a rendezvous with Colonel Hugo Hauptmann, the Luftwaffe's most decorated officer, who is an informant. Hauptmann tells Hogan that the Gestapo has discovered him. He has a map of submarine pens, but will not give it up until his life is safe. Hogan agrees to do what he can, but then they hear a noise. Hogan escapes, with promises of getting back to Hauptmann. The Gestapo burst in and arrest Hauptmann. But they do not get the map - Hauptmann has hidden it on his window blind.
Later, Schultz tells Hogan about the arrest, and Hauptmann is being held at Stalag 13 for court martial. This is a matter of appearances as Hitler does not want people to know Hauptmann is a traitor because he is a national hero. But of course it is just a show trial with execution predetermined. Burkhalter is prosecutor and appoints Klink as defence counsel. When Hogan hears, he persuades Klink to let Newkirk help him. Klink, who has been banished from his own office until the trial ends, tells Hogan that his office is filled with the Gestapo's evidence against Hauptmann. He does not know what it is as he is not allowed to see it. And he himself believes Hautpmann is guilty.
Klink arranges for Newkirk to see Hauptmann, but Hauptmann will not tell Newkirk where the map is; he expects Hogan to help him. He says the only evidence against him are telephone recordings, which are locked in Klink's office. The bug was planted by Hauptmann's secretary, Fraulein Hibbler, who is a Gestapo agent. Hauptmann suggests that Hogan destroy the evidence. So Le Beau breaks into the office for the tapes, which are then returned after some adjustments.
Next day, the trial opens in the rec hall. Hogan starts spinning Klink a line about Hautpmann being framed by Hibbler, who he claims is a British agent, Sally Holmes. When Hibbler takes the stand, Klink accuses her of being Holmes and the tapes are fakes, but his usual incompetence hampers his strategy. However, when the tapes - and the tampering - are played, it seems to prove Klink's claim. The court has Hibbler arrested, and Hauptmann is acquitted.