Ronnie was, of course, in ignorance—as was also everyone at the Hendon Aerodrome—that Henry Knowles, the hard-working, painstaking mechanic, whose expert work it was to test machines, was not really an Englishman as he pretended to be, even though he could imitate the Cockney tongue, but that his actual baptismal name was Hermann Klitz, and his place of birth Coblenz, on the Rhine.
With wondering eyes the airman watched the mechanic pass into the dark, silent house.
“Very strange!” he remarked beneath his breath. “Very strange indeed!”
But his curiosity was increased by the arrival, ten minutes later, of a rather short, middle-aged man of distinctly burly build. The newcomer hesitated for a few minutes, gazing about him furtively, as though he feared being followed, and then slipped through the gate up to the house, where the door fell open, he being apparently expected.
“Did you see that man, Cranch?” asked Pryor in a whisper. “That’s Germany’s great spy—Mark Marx. He’s been in America for the past ten months or so, and is now back here upon some secret mission concerning our aircraft—upon which he’s an expert.”
“They’re holding a council here—by the look of it,” remarked the detective. “Five of them have gone in—and why, look! Here comes another—a lame man!”
“Yes,” said Ronnie. “This secret place of meeting is known to the spies of Germany as ‘Number Three.’ From here certain of the clever activities of the invisible hand of Germany are frequently directed, as from other centres; Mark Marx is a clever adventurer who used to be the assistant director of the enemy’s operations in this country. Apparently he has returned to London to resume his sinister activities against us. He acts directly under the control of the head of Germany’s secret service in this country, that shrewd, clever, and influential person who hides his identity beneath the official description of ‘Number Seven.’”
“Then ‘Number Three’ is the headquarters of ‘Number Seven’—eh!” asked the ex-detective in a whisper.
“Exactly. That some devilish conspiracy is now afoot is quite certain. Our duty is to discover and to thwart it. I was secretly warned that Mark Marx had returned, and now, knowing that it is so, I must take adequate precautions.”
“How shall you act?”