Because the Nabokovs made it clear that they altered the novel to a significant degree, I'm not sure how much of what I read really dated to the 1920s. I assumed the English version was faithful in the small details to the original, and I enjoyed the little everyday details of life in what would retroactively be called Weimar Germany.
The plot, in broad strokes, is as follows. A country boy comes to Berlin, to work in his uncle's department store. The nephew and the uncle's wife really hit it off, to the point that they begin a steamy love affair, which the uncle remains cheerfully oblivious to.
Queen and Knave conspire to murder the King, despite both being quite inexperienced at planning and carrying out a crime of this magnitude. Psychological tension and fatal complications ensue.
The plot, as the author readily admits, is not terribly innovative. It's the prose (even if the reader is never quite sure if any given snippet of English prose came from the elder or younger Nabokov) and the macabre atmosphere that make the novel worth reading.
I never trained myself to intelligently discuss prose the way a literature professor might, so I feel somewhat hobbled when talking about modern literary writers like Nabokov.
To me, King Queen, Knave provides a look at life in 1920s Germany. It lets me play at deducing what was the same in the original 1928 Russian-language edition, and what was changed in the 1960s. But it's been two months since I actually read the thing (while traveling in southern Turkey in late August), so I can't quite recall the prose enough to discuss it at any length.