"Quite," the colonel smiled. "But don't bother, because I'll explain. All this certainly must sound like so much gibberish to you two. Well, Number Sixteen Kholerstrasse, in Duisburg, Germany, was for a long time the Western Germany Headquarters of Himmler's Gestapo, and the German Intelligence, and secret police. The clearing house, you might say, for all of the occupied countries, as well as the British Isles. Well, a few months ago we got wind of its existence. Also word that Himmler himself, and several of his important key men, were going to meet at Number Sixteen--for a little discussion of policy, no doubt. Anyway, it gave us a very bright idea, though the idea failed to turn out one tenth as bright as we had hoped. To make a long story short, we formed a sort of Commando squad of British, American, and Russian agents operating in Germany, plus a few members of the French Underground. The idea was, of course, to storm the place and wipe out Herr Himmler, and several others, and perhaps capture valuable papers and such." The colonel paused for a moment, and a look of bitterness and sadness came into his eyes. "The raid was made, but luck was not with us," he presently continued in a low voice. "Something went wrong. Nobody seemed to know just what, or why. But it appeared that the Huns had been tipped off. Neither Himmler nor his key men were there at the time of the raid. Our men killed a few of the lesser lights that were present. But five of our men died, and not one slip of paper of any value was obtained. Hand grenades and rifle fire made a mess of the place, so the report stated. But that's about all the raiding party accomplished. It was just another one of those rotten bits of luck that couldn't be helped. Like the time the Commandos raided Marshall Rommel's Headquarters in Libya, only to find that the Desert Fox had flown to Germany the day before for a meeting with Hitler. No, we are not very proud of the Sixteen Kholerstrasse affair. The Hun beat us at every turn that time. And now the beggars are obviously using it again. _That_ is indeed interesting." "And four of those mysterious factories are in the Duisburg area," Major Crandall murmured. "Don't forget that, sir. I'd like to make a bet that Number Sixteen is the nerve center of whatever the Nazis are cooking up for us." "And I wouldn't take that bet, Major, because I quite agree with you," Colonel Fraser said firmly. "And here's one more thing. Not only has Herr Baron underscored the date when the last agent is to report to Number Sixteen, he has also referred to it as Der Tag. The big day for whatever it is they are preparing." "And it will be a blow against the combined American and British air forces in England," Freddy Farmer murmured. "Unless, sir, there's more in the little black book than you've told us?" Everybody looked at young Farmer in puzzled surprise.