Crouched behind the packing case, Dick waited, wondering what was to come next. Now that he was here, he felt that he had done a foolish thing, and one only likely to lead to more trouble. There was so little chance for him to accomplish anything of value, and he was not even sure that Hallo would come here at all. Perhaps he was going somewhere else, and he had simply walked into a trap without even a chance of getting any results.

Yet luck had been with him so far in good measure. He had been almost marvelously lucky in the boathouse. That Hallo had not seen him there had been due only to fortune, and scarcely at all, in the first perilous moment, to any action of his own. And he had been lucky again in his trip into the city with Hallo ahead of him. In war time, as he knew, people were likely to be suspicious of any stranger, even of one acting in the most ordinary fashion. And his behavior, as he dodged and trailed after Hallo, had been more than suspicious.

And finally, too, his getting into Hallo's office and finding a place to conceal himself so well had been luck of the greatest sort. He had taken a wild chance when he darted ahead of his man, for he might well have found the door closed and locked, and have been caught by Hallo before he could have got away. But he had staked everything on the hazard that there ought to be a night watchman, as there would be, he knew, in any American warehouse. And a night watchman there was. He had seen the light of a man's lantern, as he came up to the door, from a second floor window. The door had been open, and so he had slipped in.

The very fact that the door was open, too, encouraged him. It seemed to him to make it certain that Hallo was expected some time that night. And then a sound of brisk footsteps on the uncarpeted floor just outside the office set his pulses leaping. Was it Hallo?

He could not tell, and he dared not emerge from his shelter enough to see. In a moment the room was light, but Dick was still hidden, and the movements of whoever had come in were hidden from him, too. But he was sure of one thing after a very few moments. It was not Hallo with whom he now shared the room. The newcomer's movements were too brisk, too quick, for Hallo, who was slow moving, rather heavy footed, though agile enough when it was necessary. Consumed by curiosity, Dick gradually edged over so that he might have a chance to steal a look around the edge of the big packing case without being seen himself.

He succeeded at last, and just as he looked around, the door was opened. Hallo came in, and Dick darted back—but not until he had seen that the other occupant of the room was Stepan Dushan! Now he felt that he was amply justified for what he had done. Steve was here, and between them there should be a chance to do something, he thought!

"What are you doing here?" growled Hallo, as he saw Stepan.

"I came to try to catch up with my work, sir," said Stepan, meekly. "Since I could not be here to-day during the day, I thought it would be better if I got ready for to-morrow, so that in the morning no time would be lost."

"H'mph!" growled Hallo. He hesitated for a moment; then, half satisfied, sat down at his desk.

"Did you have a fall, sir?" asked Stepan, and Dick almost choked with laughter at his tone. "You're all covered with mud. Shall I get a brush and try to take some of it off?"

"No! Attend to your work!" roared Hallo. "Or here—clear out of here! I'm going to be busy, and I want to be alone."

It seemed to Dick that Stepan was hesitating, that he was on the verge of refusing to obey, and so of giving everything away. But he yielded, after just a moment's pause.

"Very well, sir," he said. "I'll go down to the stock-room and make the tally from last night's sheets."

"All right. Be off!" said Hallo, ungraciously.

And now one thing filled Dick's mind. How much had Hallo seen or heard while he was hidden in the closet of the boathouse? Did he know Stepan's real work, and the part he was playing in these stirring times? If he did, and had concealed his knowledge, it meant that he was laying a trap for Stepan, and it meant, too, that he was a good actor, for he had managed to conceal his knowledge admirably if he really possessed it.

Nothing that Dick knew of Hallo made it seem at all likely that he could dissemble well enough to keep from betraying his knowledge, if he had it. But Dick felt that it would not be safe to assume that, because his father had trusted Hallo to a great extent, and had supposed him to be an honorable business man, and there had certainly never been anything in his conduct to suggest that he would behave as he had done. And, moreover in New York he had seemed a plodding, stolid business man, and had never seemed to have it in him to play a part in the sort of intrigue that so evidently occupied much of his time in Semlin.

For a time the room was absolutely quiet. Dick wondered where Steve had gone. He was sure, somehow, that his chum was within reach, probably within hearing, waiting like himself for Hallo to do something. And Dick guessed, too, that Steve must have discovered by this time that the boathouse was empty, and thought that perhaps it was in search of him that Steve was here. That worried him, but for the time there was nothing to be done; he could only wait. The one preparation he could make for whatever might be coming was to get very close to the edge of his shelter, so that he could with little risk peep out from time to time. Each time that he looked out he saw Hallo, head bent low over the table, writing furiously.

Then came the break in the tension. Outside, echoing on the flags, came hurried footsteps. Dick listened eagerly. They turned in and came clattering up the steps. He dared not risk a peep just then to see who was coming. For the time, he decided, his ears must do the work of his eyes as well as their own share. He heard Hallo spring up, overturning his chair.

"You! Here?" cried Hallo, in a low voice. "Are you mad, man?"

"No. I had to come," said the newcomer. "It was impossible to send word, and you had to know—someone had to know! I suppose it was risky for me to come here, but they are all at work. All except Milikoff, and I don't know where he is."

"He led a band of assassins who tried to kill me earlier in the evening," growled Hallo. "But there are others, at least two others, here somewhere. They came in this evening, from the other side."

"From the other side?" said the stranger, in amazement. "But how could that be? The river front is guarded so that a strange fly could hardly cross from Belgrade to Semlin!"

"Don't you know of the boathouse near Milikoff's place?"

Hallo's tone was suddenly menacing. Dick could imagine that he was leaning forward, pushing his heavy jaw into the face of the other man. He remembered that trick of the Hungarian's.

"Boathouse—near Milikoff's? No!" stammered the other. "I never heard of it before!"

"H'mph!" Hallo's voice expressed doubt, distrust. "Perhaps not! Well, I came upon it by chance to-night, and it saved my life. They seized me to-night when I was in conference with them—the treacherous hounds! But they are Servians! I might have expected it! I escaped, and they chased me! They chased me right into the Danube! I swam, while they peppered the water. They had those new silencers, so that it was safe to shoot. And by the merest chance, when I was nearly exhausted, I suddenly saw a door in front of me, as I was trying to swim ashore in an inlet, in the hope of escaping after I had landed. I dived, and came up in a boathouse. The door could be raised, you see, to admit a boat. There was a closet, and I hid."

"That is news to me! I never heard that there was such a place!"

"I drew the closet door to when I heard a man come in. Soon afterward a motor boat entered, and two men landed from her. I could hear their voices dully—that closet door was thick, and it was so hot and close inside that I was almost suffocated. I do not know who they were except that I think Vanya, the soldier, was the man who opened for them. Later, when they had gone, I got out and came here."

"It is good that you came. Master, it is to-night that they plan to strike a great blow. They have dug a tunnel beneath the arsenal. I am almost sure that it is to-night they have chosen to explode a great mine that will destroy the arsenal completely, with all the stores and guns it contains!"

Hallo swore savagely.

"You have done well!" he said. "Eh, there is time yet, you think, to stop them? How is it that you do not know more concerning their plans?"

"They do not trust me wholly. I know only what I can learn by spying and ferreting about. They tell me almost nothing. But I am sure that I am right. Look—I will draw you a plan that will show you where the tunnel is. I think that if you hasten you can catch most of them like rats in a trap. Milikoff is not there. He is the most dangerous of all, save one."

"Who is more dangerous than Milikoff?"

"The boy Stepan Dushan!"

"Stepan Dushan! I do not believe there is such a boy! You tell me of him, but you can never show him to me!"

For a moment Dick could not understand. But then he smiled at his own stupidity. Of course Steve was known to Mike Hallo under an assumed name; he would never have used his own, so obviously that of a Servian.

"It makes no difference whether you believe me or not, I am telling you the truth," said the newcomer with a show of spirit. "He is young, but he has done more to discover the truth than any of them. He has brought information of the concentration of the Austrian troops along the Drina. It is through him that they learned where the stores and supplies were being massed in the mountains there. I told you yesterday of the plans that were made for a Servian raid, but it was too late. A raiding column crossed the Drina last night and destroyed most of the stores you had collected."

"There has been a leak somewhere in my own place!" said Hallo, savagely. "Even the men who executed the orders did not know where those supplies were going! I trusted no one!"

"That is just the bad part of it, master," said the spy. "They will be saying next that you yourself gave away the truth to the Servians."

"Who will believe them?"

"If it is known that you have been in touch with Milikoff?"

"It is known, fool! It is known that I am treating with them simply to gain information of their plans. But now—it is no time for talk. We must move quickly, or they will explode their mine."

He lifted his voice in a sudden shout.

"Jan!" he cried. "Jan!"

And then Dick's heart sank. That must be meant for Steve—and to answer meant deadly peril for Stepan Dushan, since the spy, who had so plainly been betraying Servian secrets to Hallo, knew Stepan. Dick edged forward, waiting, wondering. Now he caught a glimpse of the spy, and then he drew back, as he saw that both Hallo and the other man had come very close to him. They stood just by the box that sheltered him.

"Jan!" cried Hallo again.

"Here I am!" said Stepan's voice. And then, "Serge!"

"Stepan Dushan! That is Stepan Dushan!" shrieked the spy.

Then, before anything more could happen, Dick had an inspiration. He threw his whole weight against the heavy packing case, straining, pushing. It trembled, gave, and then crashed down, bearing Hallo and the spy down beneath its crushing weight, sending them down, stunned and helpless.