On the day of the meeting of the British Cabinet, two men and a pretty, dark-haired French girl were keenly discussing the terrible problem in a small but tastefully furnished flat in the Avenue Kleber, in Paris.

"I know only three people in the world with brains enough to carry the thing out," said the girl. "They are Ivan Petroff, the Russian; Paolo Caetani, the Italian, and Sebastian Gonzalez, the Spaniard. They are all three avowed anarchists, and, as we know, they are all chemists and bacteriologists of supreme ability. But I must say that there is not a sc.r.a.p of evidence to connect either of them with this affair."

The speaker was Yvette Pasquet, and there was no one in whom Regnier, the astute head of the French Secret Service, placed more implicit confidence.

"If the doctors could settle whether this poisoning is chemical or bacteriological it would help us a great deal," said d.i.c.k Manton. "If it is chemical, I should be disposed to include Barakoff; he knows more about chemistry than all the others put together. But in any case, there is as yet nothing we can even begin to work on."

A fortnight went past. The death-roll in England had a.s.sumed terrible proportions, and apparently the authorities were as far off as ever from coming to grips with the mystery. But a clue came through the heroism of a London policeman.

One night Constable Jervis was patrolling a beat which led him through some tumbledown streets in the lowest quarter of Canning Town. Suddenly he caught sight of a man rushing from a small empty house. At once Jervis started in pursuit of the man, who was running hard away from him. As he did so, there came the sound of an explosion, and the house the man had just left collapsed like a pack of cards. At the same time the odour of the dreaded violet vapour completely filled the narrow street.

The Terror had attacked London, and Jervis knew that to cross that zone of vapour meant certain death.

He did not hesitate. m.u.f.fling his face with his pocket handkerchief as he ran, he dashed at full speed after the stranger, whom he could just discern. He crossed the zone of death, almost overpowered by the curious scent of petrol and musk that loaded the still air, and a moment later was in pursuit, blowing his whistle loudly as he ran. A moment later a second policeman, hearing his colleague's whistle, stood at the end of the road barring the way. The desperado was trapped.

s.n.a.t.c.hing out a revolver, the man backed against the wall and opened fire on his pursuers who were rapidly closing in on him. But both the policemen were armed, and both opened fire. Jervis's second shot killed the man on the spot.

He proved to be a well-known member of a Russian anarchist group which had its head-quarters in the slums of Soho. The gallant Jervis had faced certain death--as a matter of fact he was among the hundred or so victims when the epidemic broke out twelve days later--but he had done his duty in accordance with the splendid traditions of the force to which he belonged.

The source of the mysterious epidemic was now, to a certain extent, localised. It needed no great ac.u.men to guess the motive and origin of the fiendish plot. But to discover the master-mind which held the full solution of the mystery was another matter.

The first step was a general round-up of known members of the Anarchist Party. They were arrested by dozens, and very soon practically all who were known were under lock and key.

To the intense surprise of the police, one and all acknowledged that they were fully familiar with the scheme. Many of them had actually taken part in its execution. The secret had been well kept!

The explosions, it was learned, were caused by small bombs about the size of an orange. These were placed in the selected houses and timed to explode in a few hours. Evidently there was some defect in the mechanism of the one sent to Canning Town, and the man who placed it there must have seen that it was likely to explode prematurely and rushed in panic from the house.

But of the source of the bombs one and all of the men professed complete ignorance. They were, it was a.s.serted, received by post from different places on the Continent. It was evident that the crafty scoundrel at the head of the terrible organisation took elaborate precautions to prevent their sources of origin being discovered.

But to have traced the outbreak to Anarchist sources was a step of the first importance. Immediately every branch of the secret service of the western world was concentrated on the problem.

A hint from one of the men captured, who collapsed under the cross-examination to which the known leaders were subjected, put the police in possession of one of the bombs. It had arrived by post the day before, and the miscreant to whom it was sent was caught before he had time to make use of it.

It was now possible to prove definitely that the disease caused by the bombs was chemical in its origin. Upon a.n.a.lysis the powder with which the bomb was filled was found to consist of a series of, apparently, quite harmless chemicals. A small portion fired by the detonator found in the bomb gave off dense clouds of the pale-violet vapour, and animals exposed to it were speedily killed, exhibiting every symptom of the terrible disease. Unhappily the secret of the detonator used defied discovery. The one found in the bomb had been used in the only experiment that had been made, and too late it was discovered that no fulminating material known would explode the apparently harmless powder.

"That seems to narrow it down to Barakoff," said d.i.c.k Manton a few days later when Regnier brought them the news. "I don't think either of the others is equal to research work capable of producing such results. Do you know where Barakoff is now?" he asked in French.

Regnier shook his head.

"He was in Moscow a year ago," he replied, "and after that we heard of him in Prague, in Rome, and lastly in Madrid, but he disappeared suddenly and we have not been able to pick up his tracks again. He is a short, powerful, thickset man with a rather hunched back, but nothing else peculiar about his appearance."

Next day, however, Regnier came to the adventurous trio in great excitement.

"Barakoff is in England!" he declared. "We have just had word from Gaston Meunier who saw him in Brighton a week ago!"

"But how on earth did he get there?" asked Jules. "You know every one has been looking for him for months past. He could not possibly have got through by any of the ordinary routes."

"I'm as puzzled as you are, monsieur," was Regnier's reply.

"Well, if he is there we'd better go over," said d.i.c.k. "Yvette can go with me in Mohawk II and Jules by the night boat. I shall fly the Mohawk to my old shed in Norfolk; I have kept it on in case of emergency, and it is quite safe."

An hour later d.i.c.k was in close talk with a young Russian named Nicholas Fedoroff. He had been an active member of a circle of dangerous anarchists in Zurich, but had dropped out and was now living in Paris.

By good fortune d.i.c.k had saved his baby girl, at imminent risk of his own life, from being killed by a motor-van in Paris, hence Fedoroff was impulsively grateful.

"Look here, Nicholas," said d.i.c.k bluntly. "I want you to tell me anything you can about Barakoff."

They were seated in a small cafe in the Rue Caumartin, which was Fedoroff's favourite haunt. The Russian glanced round fearfully.

"Hush!" he said in broken French and in evident horror. "I--I can't tell you! He has agents everywhere. If I were heard even speaking his name I should never get home."

The man's agitation was so p.r.o.nounced that one or two men in the cafe glanced at him curiously. d.i.c.k saw that the mere mention of Barakoff's name had thrown the Russian completely off his balance.

"Come to my flat," he said quietly, "you have got to tell me."

They drove in a taxi to d.i.c.k's flat, where a stiff dose of brandy pulled the Russian together. Yet he still trembled like a leaf.

"How did you know that I knew Barakoff?" he asked.

Instantly d.i.c.k was keenly on the alert. He had no idea that Fedoroff had been a.s.sociated with the notorious criminal; his appeal to Fedoroff had been a chance shot. Evidently he had stumbled on a matter of importance. But he was quick to take advantage of his good luck.

"Never mind how," he said. "I do know, and that's enough. You have got to tell me. I believe Barakoff is at the bottom of the trouble in England. I know he is there, and I want to know where he is and how he got there."

The Russian's agitation increased.

"You must not ask me; I cannot tell you," he gasped.

"Then a few words from me in a certain quarter--not the police," d.i.c.k suggested.

The Russian collapsed.

"No, no, I will tell you," he moaned. "He is in England, but I don't know where. He flew over."

"Flew over!" echoed d.i.c.k in utter amazement. "Nonsense, he couldn't have got in that way. Every aerodrome in England has been watched for months."

"But he did," the Russian a.s.serted. "He has his own aeroplane. It makes no noise, and it goes straight up and down."

Here was a surprise indeed! The secret of the helicopter with its almost unlimited power for evil was also in the hands of one of the most desperate ruffians in the world! There was indeed no time to be lost.

Fedoroff could tell d.i.c.k little more. What the secret of Barakoff's influence over him was d.i.c.k could not fathom. He would say nothing, but evidently was in deadly fear.

One little item d.i.c.k did indeed extract and it was to prove valuable.

Fedoroff knew that Barakoff had a.s.sociates in Soho. And that was the only clue they could gain to his possible whereabouts.

That evening d.i.c.k, Yvette, and Jules crossed to England, and with official introductions from Regnier, d.i.c.k lost no time in getting into communication with Detective Inspector Buckhurst, one of the ablest men of Scotland Yard's famous "Special Department," a man whose knowledge of the alien sc.u.m which infested London was unrivalled. To him d.i.c.k told all he knew.

Buckhurst looked grave.

"I know of the man, of course," he said, "but I have never seen him and I don't think any of my men have. We have combed Soho out pretty thoroughly, but no one answering to Barakoff's description has been seen."