“I wonder where the uninvited guests are?” asked Tom.

“Probably in the airship shed,” answered Ned.  “Koku was after them as soon as the alarm went off, and they couldn’t have gotten away.  They must be inside there yet.  But what can their game be?”

“It’s hard to say,” admitted Tom.  “They may be trying to get something belonging to me, or they may imagine they can pick up some valuable secrets.  Or they may—­” He stopped suddenly, and then exclaimed: 

“Come on, Ned!  They’re after one of the airships!  That’s it!  My big biplane is all ready to start, and they can get it in motion inside of a few seconds.  Oh, why didn’t I hurry?” he added, bitterly.

But the hangar was still some distance away, and it would take two or three minutes of running to reach it.

Meanwhile, and at the instant Tom had his thought of the possible theft of his biggest aircraft, something happened.

The doors of the shed were suddenly thrown open, and the two boys could see the large airship being wheeled out.  The hazy light of the moon behind the clouds shone on the expanse of white planes, and on the fish-tail rudder, one of Tom’s latest ideas.

“Hey, there!” cried Tom, warningly.

“Leave that alone!” yelled Ned.

“Koku!  Koku!” shouted Tom, shrilly.  “Get after those fellows!”

“Me get!” boomed out the giant, in his deep voice.

He had been standing near the entrance to the hangar, probably waiting for developments, and watching for the arrival of Tom and Ned.  The big form was seen to leap forward, and then several dark shadows swarmed from around the airship, and were seen to fling themselves upon the giant.

“That’s a fight!” cried Ned.  “They’re attacking him!”

“Koku can take care of himself!” murmured Tom.  “But come on.  I don’t see what their game is.”

He understood a moment later, however, for while several of the midnight visitors were engaged in a hand-to-hand tussle with the giant there came a sharp, throbbing roar of the airship motor in motion.  The propellers were being whirled rapidly about.

“Koku!  Koku!” cried Tom, for he was still some distance off.  “Never mind them!  Don’t let the airship be taken!”

But Koku could only grunt.  Big and strong as he was, half a dozen men attacking him at once hampered him.  He threw them from him, one after another, and was gradually making his way toward the now slowly-moving airship.  But would he be in time?

Tom and Ned could not hope to reach the machine before Koku, though they were running at top speed.

“Koku!  Koku!” yelled Tom.  “Don’t let them get away!”

But Koku could only grunt—­harder this time—­for he fell heavily, being tripped by a stick thrust between his legs.  He lay for a moment stunned.

“They’re going to get away!” panted Tom, making an effort to increase his speed.