Hogan and Co successfully blow up a convoy, the latest in a series of jobs. However, the frequency of the sabotage is making the Gestapo suspicious. Major Hochstetter pays a visit to Klink and tells him something he has noticed about the attacks - Stalag 13 is geographically located in the center of them. Klink cannot believe Stalag 13 is a center for sabotage, but Hochstetter says he is going to keep an eye on the camp. And the Gestapo is also increasing security and radio monitoring. Naturally, Hogan and Co have been eavesdropping on the coffee pot. Hogan gives the order to lie low, although they have a munititions train job scheduled for the following week.
The Gestapo delivers a new prisoner to Stalag 13, Captain James Martin. As with all new prisoners, Hogan has him checked out. Martin passes the screening, so Hogan shows him their operation, including the plan to blow up the munitions train (now back on schedule). Martin volunteers to join the mission.
Then Martin assaults Schultz at roll call and is taken to Klink's office. There he hands Klink a written note. Klink's attitude suddenly softens and he lets Martin off. Hogan and Co are listening on the coffee pot. They have no visual for seeing Martin hand Klink the note, but they are suspicious at the ease in which Martin got himself out of trouble. Hogan gets even more suspicious when a Gestapo officer snaps to attention when Martin passes - something he is not supposed to do with an enemy soldier. Hogan orders Le Beau to get him the list of recent transfers out of camp. He discovers that a sergeant from Martin's squadron was recently transferred out of camp - suggesting it was to silence him. Hogan announces to the others that Martin is a phoney. And Martin is indeed a phoney; the note he passed to Klink said that he is really Captain Seifert of the Gestapo on special assignment. Seifert has Klink phone Hochstetter and tell him to come to Stalag 13 on Wednesday night (the night for the train job).
Hogan has Seifert tied up and kept under guard. The plan is to pull off the train job and plant Seifert's jacket, which they have purposely scorched up to make it look like Seiffert broke camp to go after the saboteurs on his own, but was caught in the explosion. In reality, Seifert will be sent to a POW camp in England.
Hochstetter arrives and discovers Seifert missing. He is furious when he receives a phone call that the munitions train has been destroyed; this could mean the end of his career. He also points out to Klink that is the first successful escape from Stalag 13. Klink replies that he does not count it as official because the man was a plant. Hochstetter finds Seifert's scorched jacket and assumes what Hogan hoped he would.