As Mr. Cook left the room the two boys heard the automobile come up the driveway and stop in front of the house. Mrs. Cook and Louise were to spend the evening with an aunt of Bob's a short distance down the street, and Mr. Cook was to take them there in the car. Bob and Hugh waited until they should all leave for they did not want to be seen by any one in their disguises.

Presently they heard the car start off and they knew the coast was clear. Silently they slipped down stairs and out the front door. By the side of the house they paused for a consultation.

"These whiskers itch awfully," exclaimed Bob.

"So does this mustache," said Hugh. "I guess we'll have to endure it though."

"Where shall we wait?"

"Won't Lena come out the back door?"

"I guess so. At any rate she'll have to come around and go down the front walk, there's no other way for her to get out of the yard."

"Let's cross the street and wait there then."

They followed that plan and presently were standing side by side in the shadow of a tree on the opposite side of the street. Lena could be expected to appear at any minute and they kept a sharp lookout for her.

"What do you suppose is ahead of us to-night?" asked Hugh in a low tone.

"I wish I knew."

"I hope we aren't going off on a wild goose chase."

"You've been saying right along that we ought to watch Lena," Bob reminded his friend.

"I know that and I think it's a good plan. All I say is that she may fool us in some way if we're not careful."

"How do you suppose Mr. Wernberg's getting along in the hospital?"

"I don't know," said Hugh. "I must say though that I'm more interested in Lena."

"I'd like to see our old friend, the false detective."

"So would I. What do you suppose he is—"

"Ssh," hissed Bob suddenly.

Around the corner of the Cooks' house came a woman. She walked briskly and a moment later had reached the street. She gazed apprehensively up and down while the two boys shrank farther back into the shadow; then she started off in the direction of the city's business district.

"That's Lena," whispered Bob. "Come on."

On the opposite side of the street and perhaps a hundred paces in back of the hurrying woman the two boys followed.

"We'll have to keep closer than this when she gets down town," whispered Hugh.

"I know it," agreed Bob. "She'd get suspicious now though."

Now and again Lena stopped and glanced behind her. Every time she did so the boys stopped too; evidently she was afraid of being followed. They met few people and those who did pass them apparently took them for a couple of tramps, for they paid no particular attention to them.

A little distance down the street Lena turned the corner to her right. The two boys as a consequence had to run in order not to lose sight of her. They were fearful lest she should slip away from them and therefore were greatly relieved when they came to the turn and saw her still in front of them.

A few moments later she turned again, and then presently, turned still a third time.

"She's trying to lose us," whispered Bob.

"Maybe not," said Hugh. "This is Elm Street."

"Where's twelve eighty-two!"

"On the next block."

The white stucco house was on the same side of the street with the boys, and as Lena came opposite it she crossed over. Bob and Hugh stopped short under a large maple tree whose trunk cast a shadow affording ample protection from a nearby arclight. From this vantage point they watched the woman they were trailing.

"She's going in," whispered Bob, clutching Hugh's arm excitedly.

Lena turned in from the side walk and started toward the steps of the white stucco house, number twelve eighty-two. Half-way up she paused irresolutely. She acted as if she was puzzled as to what she should do; finally she turned, descended the steps rapidly and continued on down the street.

"That was queer," whispered Bob.

"It looked as though she lost her nerve."

"Why should she be scared to go in where her gang is!"

"Don't ask me. Come on."

Once again they took up the chase. Lena seemed to walk more swiftly than ever now, and it was not an easy task to keep pace with her and still not be seen. The night was dark with low-hanging clouds, the street lamps affording the only light available. Ahead they could see the reflection from the lights of the main street of the city.

"Do you suppose she dropped a note or anything on that porch back there?" demanded Hugh suddenly.

"I didn't see her do anything like that," said Bob.

"Nor I. At any rate I guess the best thing we can do is to stick close to her."

"Yes, and we'd better keep closer too, now that we are coming to where the stores are. We'll lose track of her if we don't."

"Do you suppose any one will notice that we're disguised?"

"I hope not. There's usually a big crowd on the streets Saturday night though."

"We'll hope for luck," said Hugh earnestly.

They quickened their paces until they were scarcely more than seventy-five feet in back of Lena. There were many people passing them in both directions now, and apparently Lena was not as suspicious as she had been; she glanced behind her no more.

Presently they turned into the main street. The sidewalks were thronged with people and everything was lighted up brilliantly in the glare of arclights and shop windows. Lena was just ahead of the boys and it was not an easy task to follow her in the crowd.

Music sounded down the street. A troop of cavalry was approaching and every one lined the curb to see them pass. Lena stopped and the boys took their places directly behind her. Every trooper was mounted on a coal black horse, and they made a fine showing as they drew near; the crowd began to cheer and many waved small American flags that they were carrying. Women waved their handkerchiefs as the horsemen passed, and much to both Bob's and Hugh's surprise Lena waved her handkerchief and clapped her hands with the others.

"What do you think of that?" whispered Bob.

"Bluff," said Hugh. "She's clever."

The crowd began to break up and presently was moving up and down the street again. Lena started on her way once more, and almost at her heels followed Bob and Hugh. They were beginning to wonder whether they were following a false clue. It might be that Lena had dropped a message on the porch of the house on Elm Street, and if so her work was probably done and there could be no object in following her farther.

Suddenly Hugh seized Bob by the arm. "Look at this man coming," he hissed.

Not thirty feet distant and walking directly toward them was the false detective. There could be no mistaking him. Bob and Hugh, forgetting for the moment that they were disguised were fearful lest he should recognize them as well. A moment later, however, an interesting event happened right before their eyes, and they forgot all else.

As the "detective," the man with whom they had fought that morning, the man who had blown up the deserted house, and whom they suspected of having tried to blow up the railroad bridge in the afternoon, passed Lena he held a slip of paper in his left hand. As she went by she took it with her left hand, though as far as the boys could see the two conspirators had not even looked at each other.

Lena continued on down the street as if nothing had happened, while the detective also kept on as though unconscious of having seen Lena at all. He passed the two boys without even a glance.

Bob and Hugh stopped short.

"What do you think of that?" demanded Hugh. "What'll we do?"

"Follow them," said Bob quickly. "You follow him and I'll trail Lena."

Without another word the two boys separated.