PROMPTLY at eight o'clock the next evening Mr. John Stapleton left his house in the Avenue Kleber, in a big French touring car, with FranÇois at the wheel.

The car presented no points of peculiarity, being like a thousand others to be seen any evening upon the streets of Paris. It was of large size, high powered, and painted a green so dark as to be almost black.

Mr. Stapleton sat in the tonneau, wearing a dark blue serge suit, and a Panama hat. In his left hand he clutched a small package, about the size of a cigar box. In the package were banknotes amounting to one hundred thousand dollars.

Close beside his right foot lay a rubber bulb, from which a short pipe extended through a hole bored in the side of the car. The end of the pipe held a small brass nozzle. It projected but a short distance beyond the body of the car, and in the dim light of early evening was quite invisible.

Mr. Stapleton told his chauffeur to drive out the road toward Versailles. "I feel like getting some fresh air," he added. "It's rather warm, tonight." Inwardly he was burning up with excitement.

From Paris to Versailles is a matter of some fourteen miles. Mr. Stapleton's car proceeded slowly. He wanted to run no chances of missing the car with the blue light.

At the Porte de Versailles he paused long enough to see Richard Duvall, standing in the shadow of the gateway. Then he passed outside of Paris.

There were many automobiles and other vehicles on the road. The evening was a pleasant one, and all Paris seemed out taking the air. The majority of the vehicles were coming toward the city. He observed a car, some distance behind him, containing a single occupant, a man of middle age, but paid no attention to it. His eyes were strained to detect in the cars approaching him some evidence of the signal light which was to rouse him to sudden action.

He noticed that FranÇois, like himself, was carefully scrutinizing each car as it approached them. He wondered if the chauffeur could have any idea of the purpose of his expedition; but presently dismissed the thought as entirely unlikely, and devoted himself to the passing cars.

He had proceeded perhaps four or five miles beyond the fortifications, when he saw a large car approaching slowly from the direction of Versailles. It contained but two persons, the chauffeur, and a heavily veiled woman.

The chauffeur, who was keenly observing the machine in which Mr. Stapleton sat, began to swerve to the right side of the road, so as to pass as closely to the banker's car as possible. At the same moment there showed through the gathering darkness a brilliant spot of blue light in the tonneau where sat the woman.

Mr. Stapleton was on his feet in an instant. The two cars approached each other rapidly. It was necessary for him to act with great quickness. He shifted the package containing the money from his left hand to his right, and a moment later had tossed it lightly into the other car.

He saw at once that it landed safely within, and at the same instant he pressed his foot down hard upon the rubber bulb. In a moment the car with the blue light had swept past, and was disappearing rapidly in the direction of Paris.

Mr. Stapleton leaned forward and addressed FranÇois in a voice which quivered with excitement. "Drive home at once," he commanded.

In a moment he was following the first car toward the city.

He did not notice, as he swept down the darkening road, the car which had been following him all the way from Paris. It continued on its way toward Versailles. In it were two people. At the wheel sat a man who bore, in the semi-darkness, a striking resemblance to FranÇois, Mr. Stapleton's chauffeur, while in the rear sat a figure, in dark suit and Panama hat, which seemed for all the world like that of the banker himself. Had a casual observer not seen Mr. Stapleton turn back toward Paris, he would have concluded that he was still on his way toward Versailles.

The occupants of this second car also appeared to be keenly watching the various automobiles which passed them, as though expecting some signal, some recognition; yet, in spite of their eager and expectant glances, they seemed doomed to disappointment.

At last Versailles was reached. The elderly man in the tonneau gave a short command, his chauffeur turned the car about, and they began to return to Paris. Nothing further whatever happened on the Versailles road.

Meanwhile, Richard Duvall, at the Porte de Versailles, was carefully scrutinizing the various incoming machines that passed the gate and entered the city. With a brilliant electric searchlight he examined their bodies and wheels, looking always for the telltale red stains which would identify the kidnappers' car. Beside him stood Vernet, one of the Prefect's assistants, with whom Duvall had become well acquainted during his former stay in Paris.

"Well, Monsieur Duvall," remarked the latter, "a most ingenious plan—this of yours. I wonder if it will be successful?"

"I feel sure of it."

"I hope you are right." He looked at his watch. "Half past eight. About time, I should think, from what you tell me. Here is a big fellow, now. A Pasquet, by her looks. Six-cylinder, too."

Duvall glanced at the oncoming car. A wagon which preceded it was just passing the gates. The big Pasquet slowed up, and almost stopped.The detective threw the rays of his searchlight on the body of the car, then started back with an exclamation. From one end to the other, the dark green finish of the sides and wheels was spattered and streaked with bright red paint. Dust had settled in it, in places, especially on the wheels; but above, on the doors, it was clear and unmistakable.

"Vernet," he shouted, excitedly, "it is the one! Quick! Don't let them get away."

Vernet stepped up to the quivering motor. At the wheel sat a young man, quite composed. In the tonneau, a veiled woman reclined at ease. In her hands she held a brown paper package.

She leaned toward Vernet, and spoke a single word to him. Duvall did not hear what it was; but its effect upon the Prefect's man was instantaneous—electrical. He stepped back and raised his hat. "Pardon, Madame," he said, and the Pasquet rolled through the gate and into the streets of Paris unmolested.

Duvall had sprung forward, and, as he did so, swept the occupants of the car with his electric searchlight. Suddenly he drew back in amazement, just as Vernet allowed the car to pass on. He could scarcely believe that what he saw was a reality. There was the big black car, its body and wheels plentifully bespattered with the identifying red stain—and there, at the wheel, sat Alphonse Valentin, while the veiled woman in the rear was—Grace!

He did not know it was Grace—he did know that it was the woman who had been with Valentin in his room, who had brought the message from the kidnappers to Mr. Stapleton, who, in some far off and intangible way, reminded him of Grace.

There she sat, in her hand the package containing Mr. Stapleton's money—and Vernet doffed his cap to her, and permitted her to go on! Was this woman, then, hoodwinking even the police?

He sprang to Vernet's side. "Stop them!" he cried, in a hoarse voice. "They are the ones I am after."

Vernet shook his head. "Impossible, Monsieur. They are given safe conduct by Monsieur the Prefect himself."

"But—they are thieves—kidnappers!"

Vernet shrugged his shoulders. "It may be so, Monsieur Duvall; but my orders are to let them pass."

The detective ground his teeth, helpless. His scheme for identifying the criminals had worked perfectly. He had found them, only to see both them and Mr. Stapleton's hundred thousand dollars as well slip quietly through his fingers. He cursed the whole police force of Paris roundly, in his anger.

The arrival of another car distracted his attention. It was Mr. Stapleton, hurrying home, in the hope of finding his boy. Duvall did not stop him. The banker was evidently thinking of nothing but his lost son.

Several other cars passed. Duvall had no interest in them. He was about to turn away, with the intention of hunting up Mr. Stapleton and learning whether or not the boy had been returned to him, when he heard a familiar voice calling him by name. He turned. It was Monsieur Lefevre, in a big dark green car.

"Mon Dieu! Duvall!" the Prefect cried, in pretended surprise. "You here! In Paris! Or do my eyes deceive me?"

The detective looked a bit sheepish. He realized that in not calling on his old friend before now, he had been guilty of an apparent rudeness which Monsieur Lefevre might justly resent. "Monsieur," he cried, "it is indeed I." He put out his hand, and grasped that of his old chief warmly. "A little matter of business brought me to Paris. I have only just arrived."

"Indeed." The Prefect's eyes twinkled. "I hope, my dear fellow, that your other engagements will permit you to come and see me before long."

"I shall come this very evening, Monsieur. In fact, I have a matter of the utmost importance to discuss with you. Shall you be at liberty?"

"In an hour, mon ami. Until then I have other things to occupy me. Come to the Prefecture in an hour. I shall be waiting for you. For the present, adieu." He called an order to his chauffeur, and drove rapidly off into the darkness.

Duvall turned on his heel and began to look for a taxicab. "Good night, Vernet," he called out, as he went up the street.

In half an hour, he had reached Mr. Stapleton's house. He found the unfortunate banker striding up and down his library in a towering rage. "The fellows have deceived me!" he cried. "They have not brought back my boy. Did you see anything of them? Tell me!" He grasped Duvall nervously by the arm."The car into which you threw the package of money contained, besides the chauffeur, but one occupant, a woman, did it not?"

"Yes—yes! Did you get her?"

"No."

"Why not? Did your scheme to identify the car fail to work?"

"On the contrary, it worked perfectly. I stopped the car at the barrier. The woman in it had the package of money in her hand."

"And you did not arrest her! In Heaven's name, why not?"

"The police would not permit me to do so. The woman was the same one who brought you the message last night, the supposed agent of the police. They allowed her to pass the gates."

"What?" the banker fairly shouted his question. "This is ridiculous! Is the woman a criminal, or is she a detective? She cannot be both, and if she is the latter why was she in that car, with my money in her hand?"

"I do not know. But I mean to find out very shortly."

"How? I'd like to know!"

"I am going to see the Prefect of Police at once."Mr. Stapleton sank into a chair, and groaned. "I had hoped to have Jack with me by now. His poor mother is distracted. Isn't there anything, Mr. Duvall, that you can do?"

"I hope to answer that question better, Mr. Stapleton, after I have seen Monsieur Lefevre. If this woman, and her companion, Valentin, are really the kidnappers, they are in Paris, and we shall be able to lay our hands on them without difficulty. If they are not, your money, at least is safe. I must leave you now; but as soon as I learn anything, I will report to you at once. Good night."

He left the house, more mystified than he had ever been in his life. From the start, this case had apparently been one in which all the clues led to absurd contradictions, or else to nothing at all.

In fifteen minutes he was at the Prefecture.

Monsieur Lefevre sent out word that he would be occupied for a few moments, and the detective sat down as patiently as possible, to wait.