(Reading The Insidious Dr Fu-Manchu. This is Chapter Sixteen and Petrie and Smith are for once taking the offensive.)
After some searching they come to a mansion with extended walled grounds. Besides it is "...a gypsy caravan. An old woman was seated on the steps, her wrinkled face bent, her chin resting in the palm of her hand." The next thing Petries knows, Smith has picked a fight with her. As Petrie joins him a man - who Smith identifies as a dacoit - bursts from the caravan and runs for the river.
He dives in* and vanishes. Petrie watches and watches for him to surface. Eventually he gives up... and someone throws a knife at him. The only person he can see is a white-clad girl on a punt.
He gets back to Smith who still has good hold of the woman; he informs Petrie that the dacoit must have disguised himself as a duck**. When they take off the wig, the old woman is revealed to be Fu-Manchu's slavegirl in disguise, because of course she is. There are a set of odd vocal calls, then the mansion catches fire.
Petrie introduces the next section by addressing the reader directly: "That I moved amid singular happenings, you, who have borne with me thus far, have learned, and that I witnessed many curious scenes; but of the many such scenes in that race-drama wherein Nayland Smith and Dr. Fu-Manchu played the leading parts, I remember none more bizarre than the one at my rooms that afternoon." Interesting!
Smith makes a few comments, but learns little other than that she is no kin of Fu-Manchu. He leaves her with Petrie. Now we get some information; she gives as her name Karamaneh and notes that she has a sister (dead) and a brother (alive and a slave and in the power of Fu-Manchu). She makes a bargain; to tell Petrie where to find Fu-Manchu if then they go and release her brother.
* The Thames in 1912, even upstream above London, would not be pleasant swimming.
** More accurately he was under the water wearing a false duck on his head to watch and breathe. Because he had that to hand, like one does. And came up with it on without alerting Petrie who was watching every ripple. This is not as well thought out as it seems, Rohmer, really.