Edgar Rice Burroughs 
The Beasts of Tarzan [EPUB ebook] 

‘The entire affair is shrouded in mystery, ‘ said D’Arnot. ‘I have it on the best of authority that neither the police nor the special agents of the general staff have the faintest conception of how it was accomplished. All they know, all that anyone knows, is that Nikolas Rokoff has escaped.’
John Clayton, Lord Greystoke—he who had been ‘Tarzan of the Apes’—sat in silence in the apartments of his friend, Lieutenant Paul D’Arnot, in Paris, gazing meditatively at the toe of his immaculate boot.
His mind revolved many memories, recalled by the escape of his arch-enemy from the French military prison to which he had been sentenced for life upon the testimony of the ape-man.
He thought of the lengths to which Rokoff had once gone to compass his death, and he realized that what the man had already done would doubtless be as nothing by comparison with what he would wish and plot to do now that he was again free.
Tarzan had recently brought his wife and infant son to London to escape the discomforts and dangers of the rainy season upon their vast estate in Uziri—the land of the savage Waziri warriors whose broad African domains the ape-man had once ruled.
He had run across the Channel for a brief visit with his old friend, but the news of the Russian’s escape had already cast a shadow upon his outing, so that though he had but just arrived he was already contemplating an immediate return to London.
‘It is not that I fear for myself, Paul, ‘ he said at last. ‘Many times in the past have I thwarted Rokoff’s designs upon my life; but now there are others to consider. Unless I misjudge the man, he would more quickly strike at me through my wife or son than directly at me, for he doubtless realizes that in no other way could he inflict greater anguish upon me. I must go back to them at once, and remain with them until Rokoff is recaptured—or dead.’
As these two talked in Paris, two other men were talking together in a little cottage upon the outskirts of London. Both were dark, sinister-looking men…