Hogan and Co are suddenly confined to barracks and not allowed to look out of the windows because the Germans are bringing in something. Newkirk manages to wangle his way out to see what is being brought in. The label translates as "chicken hawk". Hogan figures it must be a code name, and he has a feeling they are meant to be the chickens.
The regular bug in Klink's office is currently out of commission. So Hogan plants a transmitter in a vase with some wild flowers for Klink's office. General Burkhalter arrives with some plans, which are put in Klink's safe. He ordered the chicken hawks and says Stalag 13 has been chosen to test them because, among other reasons, the Allies will not bomb a POW camp. The chicken hawks are bombs that are dropped from airplanes. And what makes them so special is the aiming device on each bomb, which zeroes in on any radio transmission. This enables the chicken hawks to find enemy command posts, radar, and other things that use radio. Burkhalter is just starting to explain where the aiming device is when the wild flowers bring on his hay fever. Klink has to get rid of the flowers - and the transmitter with them.
Under pretext of cleaning, Hogan and Co try to get into where the chicken hawks are stored in order to steal one and find out how it works. But Burkhalter stops them. Soon, they are erecting the scaffold for seating the generals who are going to watch the testing. Then Hogan remembers the blueprints in the safe. But Klink has just had the lock changed, so they have to find out what the new combination is.
So Hogan asks Klink to mind his watch as there is a thief about in the barracks. But he has to literally give the watch to Klink in order to have Klink put it in the safe. As Klink opens the safe, Newkirk is at the window memorising the new code. Hogan then lures Klink out of his office so Newkirk can raid the safe. But the diversion proves insufficient; Klink comes back too soon and catches Newkirk in the act. To cover up, Hogan claims that Newkirk must be the thief; it must be some form of kleptomania as a result of his confinement. He asks that he be the one to punish him. Klink agrees.
Then Hogan and Co see Klink coming out of his office with the blueprints in his hands. Hogan asks Le Beau to make a duplicate roll to switch with the real one. They wheel in a trash can (with Le Beau inside) close to the scaffold. While Hogan distracts Klink, Le Beau makes the switch. They take photographs of the plans, and Newkirk goes into the closet (under pretext of punishment for stealing) to develop them while they switch the plans back. But Klink, in a flush of generosity, cancels all punishments and flings the closet door open, which ruins the photographs.
The generals and film crews for the tests arrive. Burkhalter has ordered all radios removed from vehicles so as not to interfere with the tests. Hogan realises he has been going about things the wrong way and now sets about simply discrediting the chicken hawks. He orders his men to plant walkie-talkies on the vehicles. When the tests begin, Hogan cuts the signal to the transmitter in the test target. The bombs hit the vehicles instead of the target. Forgetting that radios are not to be used, Klink uses the one in Burkhalter's car to order the plane to stop bombing. So Burkhalter's car is hit as well.
Hogan asks Klink for his watch back, saying that Newkirk is back to normal and returned their property. Klink gives it back. He then gets a call from Burkhalter to say that chicken hawk has been deemed a failure and all further tests have been cancelled. He also has to foot the bill for Burkhalter's car.