I am working on my query with the help of query shark and was wondering if you could clarify something for me:
As I understand it, when writing a QL, you need to explain what the main character wants, and who is thwarting them, but what if you are writing a "who dunnit" mystery and the antagonist isn't revealed until much later?
How much of the plot should I keep up my sleeve?
Not all queries follow the template.
It helps to know the rules, so you know when you need to break them.
And if the murderer can't be revealed until the end, you'll have to talk about the plot in a different way.
Much depends of course on how the plot unfolds. If you're writing a police procedural, the plot unfolds as the investigation progresses.
If you're writing a traditional, the plot unfolds as the main character proceeds.
The best way to figure out how to talk about your book is look at how similar books are discussed.
I use Amazon as a starting point.
Three of my favorite Agatha Christie novels:
The Secret of Chimneys:
Little did Anthony Cade suspect that an errand for a friend would place him at the center of a deadly conspiracy. Drawn into a web of intrigue, he begins to realize that the simple favor has placed him in serious danger.
As events unfold, the combined forces of Scotland Yard and the French Sûreté gradually converge on Chimneys, the great country estate that hides an amazing secret. . . .
Or Lord Edgeware Dies
When Lord Edgeware is found murdered the police are baffled. His estranged actress wife was seen visiting him just before his death and Hercule Poirot himself heard her brag of her plan to “get rid” of him.
But how could she have stabbed Lord Edgeware in his library at exactly the same time she was seen dining with friends? It’s a case that almost proves to be too much for the great Poirot.
Or The Hollow
A far-from-warm welcome greets Hercule Poirot as he arrives for lunch at Lucy Angkatell’s country house. A man lies dying by the swimming pool, his blood dripping into the water. His wife stands over him, holding a revolver.
As Poirot investigates, he begins to realize that beneath the respectable surface lies a tangle of family secrets and everyone becomes a suspect.
These are all from Amazon, but you'll want more recent comps or examples, so looking at the reviews in PW is a good guide too.
In short: you set up the scene and who's investigating. In an amateur sleuth mystery, one of my requirements (but this isn't industry standard) is that the sleuth have a compelling reason to investigate.