They retraced their footsteps; and even as they arrived, Hal arose from his position above the aeroplane.

“She’ll go now all right,” he said.  “All aboard!”

Stubbs, Nikol and Ivan hesitated and Stubbs protested.  Chester drew Hal aside for a moment and told him what he and the colonel had learned.  Hal wasted no further time.

“In here with all of you,” he commanded gruffly.  “We’re going right now.”

The others hesitated no longer, and a few moments later the big machine was flying swiftly toward the Serbian capital.

CHAPTER XX.

The end of A traitor.

It was two years after the outbreak of the great war that the Austro-German armies were hurled forward in a great and final effort to crush Serbia.  Since the early days of the struggle, heavy battles had been fought upon the Austro-Serbian frontier, with success first to one side and then to the other.

Belgrade, the Serbian capital, had been bombarded time after time by the great Austrian guns and once the city had been occupied by the foe.  Later, however, the Serbians had driven out the invader and reoccupied the capital.  And now, the Austrian army, reinforced by a hundred thousand Germans, bringing the total number of troops to half a million, was again knocking at the gates of Belgrade; and the Serbians, realizing the utter hopelessness of their cause unless aid arrived from the Anglo-French troops at Saloniki, were preparing to flee.

This was the situation when the aeroplane bearing Hal, Chester and their friends descended just outside the city.

Hardly had they alighted when they were taken in charge by a squad of Serbian troops.  Colonel Anderson, acting as spokesman for the party, explained their presence in a few well-chosen words and asked to be taken to the commanding officer.  There was considerable red tape to go through before the friends finally were ushered into the presence of the Serbian commander, and that worthy immediately informed them he had but a few moments to give them.

Colonel Anderson, therefore, came to the point at once.  He told him of the conversation he and Chester had overheard a short time before.

“And you say one of the men wore a Serbian uniform?” asked the general.

“Yes, sir.”

“You don’t know who he is—­you didn’t hear his name mentioned?”

“No, sir; but I would know him again if I saw him.”

“Good.  You shall have the chance.  Now, how far from the city do you say this conversation took place?”

“Must have been all of ten miles, sir.”

“Then the men have hardly returned to the city yet.  And you say you did not hear the name of the member of the general staff, the first traitor, or spy mentioned as having divulged information?”

“No, sir.”

“Very well.  Now I will leave all of you here for an hour or so.  I have some matters to attend to.  When I come back we’ll see if you can identify the man you speak of.”