(Reading the first Fu-Manchu novel and have now reached Chapter Five. The novel is out of copyright and freely available online if you want to read along.)
Smith decides to go undercover using "...guile against guile! We are dealing with a Chinaman, with the incarnate essence of Eastern subtlety, with the most stupendous genius that the modern Orient has produced." So he disguises himself as "A sort of Dago seaman..." Petrie insists on coming with him.
Briefing the police who will stake the place out Smith gives us a little more information on Dr Fu-Manchu: "I have only the vaguest idea, Inspector; but he is no ordinary criminal. He is the greatest genius which the powers of evil have put on earth for centuries. He has the backing of a political group whose wealth is enormous, and his mission in Europe is to PAVE THE WAY! Do you follow me? He is the advance-agent of a movement so epoch-making that not one Britisher, and not one American, in fifty thousand has ever dreamed of it.*"
They then head down the Thames (from Wapping towards Limehouse I think) in the police launch, in the dark, with lots of spooky sounds and lights from steamers and gasworks etc. Foreboding! They are dropped off near Shen yan's where they will presumably enter in the next chapter.
* Again, unusual phrasing. So epoch-making that very few people have imagined it? Also 1 in 50,000 suggests that there are several thousand British and American people aware of the situation. If Fu-Manchu intends to murder everyone concerned about Asian supremacy he needs to work a bit harder; he's managed three so far in the story, at the cost of one of his own.