Hogan and Co are out on a work detail when one Major Heintzen stops by. He has a flat tyre and wants it fixed. When Hogan and Co refuse, Heintzen orders them to fix it at gunpoint. But this is a cover for the message Heintzen passes to Hogan: three mobile rocket launchers have been moved into the area. He knows the general location of two of them, but Hogan and Co are to find the third one and report it to London. He says there is a spotting shed on the hill overlooking Hammelburg and one of their agents, Frankel, is there to help, but Frankel cannot operate a wireless. Heintzen cannot help because the Gestapo is onto him. So it is up to Hogan and Co.
Soon after, Hogan gets a break in finding the launcher - it is brought to Stalag 13. But Klink gets a call from the Gestapo about Hogan and Co changing Heintzen's flat and is told Heintzen is an Underground agent. Then General Burkhalter arrives and tells Klink that the launcher is staying at Stalag 13, and it will be fired at England the following day to coincide with an all-out Blitz. Klink is ordered to lay on extra security for the launcher. Hogan and Co are listening over the coffee pot. Hogan protests to Burkhalter that it is a violation of the Geneva Convention, but Burkhalter is not listening. Later, Hogan and Co report to London. They are ordered to assign a man to the lookout shack to radio the positions of the two other launchers, but have to take care of the third themselves. Hogan decides to go to the shack himself and leaves it up to his men to take care of the third launcher.
Hogan's men try to wangle their way close enough to Schultz to get close to the launcher, but Burkhalter spots them and has them driven off. At the shack, Hogan discovers Frankel is a woman, Lily Frankel. Frankel tells Hogan that the Nazis keep moving the launchers, which makes them difficult to spot. Hogan gets a message that the Allied bombers have taken off and need the locations of the launchers. Then Baker reports their failure to Hogan and says the rocket is too heavily guarded. Hogan asks for time to think.
Frankel tells Hogan that the rockets are controlled by gyroscopes. If the gyroscopes are altered, the rockets will be thrown off course. Hogan realises an electromagnet would do the job, but it has to be close enough. He starts reporting to Baker, but a Gestapo radio detector unit picks up their signal. The Gestapo reports this to Klink, and Burkhalter orders a search. But of course Hogan's men overhear this on the coffee pot.
Hogan comes up with a plan to deal with the third launcher, and assigns his men to do a little decoy work. So, under pretext of cooking for Schultz, Le Beau and Carter set up an electromagnet near the launcher, which is disguised as a stove. Meanwhile, Hogan and Frankel spot the other two launchers - they are coming up the road. They also spot the radio-detector unit, which is coming from the other direction. Newkirk and Baker sneak out and set up a decoy transmitter to divert the radio detection unit while Hogan reports the location of the two launchers, and the bombers destroy them. When Burkhalter launches the third, it hits his house instead of England.