Only three cabs were on the rank, and, as we entered the first, something hissed past my ear, missed both Smith and me by a miracle, and, passing over the roof of the taxi, presumably fell in the enclosed garden occupying the center of the square.

“What was that?” I cried.

“Get in—­quickly!” Smith rapped back.  “It was attempt number one! 
More than that I cannot say.  Don’t let the man hear. 
He has noticed nothing.  Pull up the window on your side,
Petrie, and look out behind.  Good!  We’ve started.”

The cab moved off with a metallic jerk, and I turned and looked back through the little window in the rear.

“Someone has got into another cab.  It is following ours, I think.”

Nayland Smith lay back and laughed unmirthfully.

“Petrie,” he said, “if I escape alive from this business I shall know that I bear a charmed life.”

I made no reply, as he pulled out the dilapidated pouch and filled his pipe.

“You have asked me to explain matters,” he continued, “and I will do so to the best of my ability.  You no doubt wonder why a servant of the British Government, lately stationed in Burma, suddenly appears in London, in the character of a detective.  I am here, Petrie—­and I bear credentials from the very highest sources—­because, quite by accident, I came upon a clew.  Following it up, in the ordinary course of routine, I obtained evidence of the existence and malignant activity of a certain man.  At the present stage of the case I should not be justified in terming him the emissary of an Eastern Power, but I may say that representations are shortly to be made to that Power’s ambassador in London.”

He paused and glanced back towards the pursuing cab.

“There is little to fear until we arrive home,” he said calmly.  “Afterwards there is much.  To continue:  This man, whether a fanatic or a duly appointed agent, is, unquestionably, the most malign and formidable personality existing in the known world today.  He is a linguist who speaks with almost equal facility in any of the civilized languages, and in most of the barbaric.  He is an adept in all the arts and sciences which a great university could teach him.  He also is an adept in certain obscure arts and sciences which no university of to-day can teach.  He has the brains of any three men of genius.  Petrie, he is a mental giant.”

“You amaze me!” I said.

“As to his mission among men.  Why did M. Jules Furneaux fall dead in a Paris opera house?  Because of heart failure?  No!  Because his last speech had shown that he held the key to the secret of Tongking.  What became of the Grand Duke Stanislaus?  Elopement?  Suicide?  Nothing of the kind.  He alone was fully alive to Russia’s growing peril.  He alone knew the truth about Mongolia.  Why was Sir Crichton Davey murdered?  Because, had the work he was engaged upon ever seen the light it would have shown