e plan of the general staff. Their reasons I cannot tell
you; but at a guess I should say it is because they hope that, by a
juncture with the Anglo-French forces, they may hope to show an effective
front until Italy can throw an army to their support, or possibly until
the long expected Russian offensive materializes."

"Then we shall have to bring some pressure to bear upon Greece," said the
younger man. "We cannot permit that. Bulgaria must get in the game sooner
and thus foil such a plan."

"Well, you probably know best," said the officer, "but remember one
thing. To all intents and purposes, Bulgaria is still neutral.
Announcement that she has decided to cast her lot with the Central
Powers, if premature, undoubtedly would spoil many plans. Particularly,
if it came to the ears of the Anglo-French commander at Saloniki."

"Exactly," replied the young man. "Our plans now are to permit the Allies
to advance a considerable distance toward Belgrade, and then to have
Bulgaria declare war at the psychological moment."

"A good plan, that," returned the officer. "But I must get back now. My
absence will be noticed and I do not care to arouse suspicion."

The men moved off.

Chester and Colonel Anderson gazed at each other.

"Rather neat little play," said Colonel Anderson.

"Rather," repeated Chester dryly.

"And to think," continued Colonel Anderson, "how leniently Bulgaria has
been treated by the Allies. Well, her day of reckoning will come."

"We'll have to get word of this to the Serbian commander in Belgrade,"
said Chester.

"So we will," said the colonel. "And also to the commander of the
Anglo-French forces in Saloniki."

"Let's get back then and see if Hal has the machine fixed so she'll fly."

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 Se