The Enigma machine is to be smuggled to England in the midget submarine. René is told to put the machine into a wine barrel and drop it into the sewer. At a certain time, everyone in Nouvion will flush their toilet and the resulting surge of water will send the wine barrel along the sewer to the submarine. The British lay on an air raid as a diversion, but they manage to bomb the pumping station by mistake, so there's no water to flush the toilets with. It's discovered that Monsieur LeClerc has put the wrong box in the wine barrel. The Enigma machine is still hidden in the café.