We all know the first line of a query is your "hook." I call the last line the "sinker." You want it to punch them in the face, in a nice, friendly kind of way that makes them unable to forget you after having read the 300 other queries in their inbox.
If you're looking for query advice, but are slightly intimidated by my claws, blade, or just my rolling googly-eyes, check out the query critique boards over at AgentQueryConnect. This is where I got my start, with advice from people smarter than me. Don't be afraid to ask for help with the most critical first step of your writing journey - the query. My comments appear in green.
Suzy wanders the streets of Hong Kong, her past in tatters, when a beaten woman hiding in an alley bring(s) back memories of an abusive childhood and a sister she failed to protect. She attempts to help, but when the woman is murdered in front of her, Suzy barely escapes with her life. That's when her nightmares return with a vengeance. I'm confused about the sequence of events here - when you say the woman she sees is "beaten" I assume she's already escaped her attacker. But if she is subsequently murdered right in front of Suzy, is it more like she's "being beaten?"
When the prostitution ring responsible tries to eliminate her as a witness, Suzy decides to fight rather than hide. She trolls the city's underbelly, human bait in a tight dress, getting answers with a knee to the groin and a call to the wife. Ha ha - great line. Her role as a prostitute hopefully you mean "role" as in she is pretending to be one, rather than actually fulfilling the position - perhaps it'd be better to rephrase as "act?" isn't doing her nightmares any favors, but that doesn't stop Suzy from discovering the dead woman was a model forced to blackmail a U.S. Senator on a junket. With a million dollars on her head and half of China on her ass, Suzy realizes a secretive organization is using sex to manipulate men in power—and she's going to do whatever it takes to end their abuse of women. Nice - I like the concept and motivation here.
After the Senator's car explodes, she has all the ammunition she needs to travel to America and lure the gang into the claws of an angry CIA. But if Suzy is to survive the coming bloodbath, she must bury her past, forget her fears, and become the nightmare others dread.
BLACK RAIN is a 100,000-word suspense novel set during Hong Kong's monsoon season, where the black rain alert can shut down the city. I love your concept, but I wonder if the word count might be a little heavy. See if you can pare down before submitting. Make sure that the ms is in as good of shape as this query is! Lastly - I'm curious about what role the black rain plays in the plot?