(Chapter Seventeen of The Insidious Dr Fu-Manchu also published as The Mystery of Dr Fu-Manchu and our protagonists are following up a tip they've been given.)
They discover Fu-Manchu in a room fitted out as a laboratory; a laboratory that matches Petrie's dream-vision from earlier. Uncanny! Smith covers Fu-Manchu, but when he orders Petrie to tie him up a dacoit threatens him with a crescent knife. Fu-Manchu gloats a little. "You supposed that I was alone? So I was***. But my faithful servant followed you. I thank him. The honors, Mr. Smith, are mine, I think?"
Fu-Manchu makes a rather complex suggestion by which they might all survive the situation, accepting Smith and Petrie's words of honour and they leave, somewhat awkwardly, up the ladder. The dacoit throws down his knife, and Smith has Petrie search him and also Fu-Manchu****. Then they let them go.
Smith immediately sheds his coat, collar and waistcoat, pocketing his valuables, declaring that they shall have to run for their lives. "We live in a peaceful age, wherein it falls to the lot of few men to owe their survival to their fleetness of foot.*****" Smith abandons his revolver ('Keep your word, though it break your neck!'), reflecting that at least it won't be used against them as Fu-Manchu prefers silent methods. They hear the calls of (three) dacoits around them as they run off to the north******. "I had never in my younger days been a notable runner; for Smith I cannot speak. But I am confident that the next half-mile was done in time that would not have disgraced a crack man."
They make it to an empty cottage. Smith breaks the latch coming in, but Petrie manages to bolt the door just in time for a dacoit to stab through it. Another breaks the window. The a grey figure appears from the shadows and shoots two of them (it turns out there were four dacoits in total). Petrie recognises the coat; it is his and being worn by Karamaneh who has saved them (again). Having failed, she goes back to him, and they leave.
* They note that his previous bases, Singapore Charlie's and the Windsor mansion have also been on the Thames; clearly using river traffic to move about.
** I can't find any evidence of this.
*** Fu-Manchu fails to count the marmoset on his shoulder amongst his companions. Losing sympathy for this cruel and murderous manipulator.
**** Something I've noted that Fu-Manchu should really have done when he last had them at his mercy.
***** This is the second time they have had a long fast run to complete. It seems to me that some track training might be in order.
****** Therefore they are on the north bank of the Thames, in Essex.