“Say, how can a town of that size,” Chalmers enquired, “be termed a mystery city in any sense of the word?  Travelling’s free in Russia.  I guess any one that wanted could take a ticket to Kroten.”

“A good many do,” Jesson assented calmly, “and some never come back.  America and Russia are on friendly terms, yet two men in my branch of the service—­good fellows they were, too—­started out from Washington for Kroten six months ago.  Neither of them has been heard of since; neither ever will be.”

“How’s it done?” Chalmers asked curiously.

“In the first place,” Jesson explained, “the city itself stands at the arm of the river, in a sort of cul-de-sac, with absolutely untraversable mountains on three sides of it.  All the roads have to come around the plain and enter from eastwards.  There is only one line of railway, so that all the approaches into the city are easily guarded.”

“That’s all right geographically, of course,” Nigel admitted, “but what earthly excuse can any one make for keeping tourists or travellers out of the place if they want to go there?”

“That is perhaps the most ingenious thing of all,” Jesson replied.  “You know that Russia is now practically a tranquil country, but there are certain bands of the extreme Bolshevistic faction who never gave in to authority and who practically exist in the little-known places by means of marauding expeditions.  The mountains about Kroten are supposed to have been infested by these nomadic companies.  Whether the outrages set down to them are really committed or not, I don’t suppose any one knows, but my point of view is that the presence of these people is absolutely encouraged by the Government, to give them an excuse for the most extraordinary precautions in issuing passports or allowing any one from the outside world to pass into the city.  If you get in, I understand you are waited upon by the police within half an hour and have to tell them the story of your past life and your future intentions.  After that you are allowed to go about on parole.  If you get too inquisitive, you are discovered to be in touch with the robber bands, and—­well—­that’s an end of you.”

“A nice, salubrious spot,” Nigel murmured.

“It sounds most interesting,” Maggie declared.  “I think a woman would be less likely to cause suspicion,” she added hopefully.

“Utterly out of the question,” Jesson pronounced.  “Kroten is the one place that must be left in my hands.  I know more about the getting there than any of you, and I know the tricks of changing my identity.”

“I should rather like to go with you,” Nigel confessed.

“Impossible!” was the brief reply.

“Why?”

Jesson smiled.

“To be perfectly frank,” he said, “because you are developing an interest in the one person in the world who might give success over into our hands.  It is necessary for you to remain where you can encourage that interest.”