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.