This one picks up some time after the end of Guards, Guards.
Sam Vimes, still Captain of the Night Watch, is also engaged to marry
Lady Sybil Ramkin, dragon-fancier and one of the wealthiest people in
Ankh-Morpork. Carrot, watchman and rightful king of Ankh-Morpork, is now
a corporal, and has come to know the city very well. This is a
good thing, as the steady influx of trolls and dwarves to the city is
fueling an equally steady rise in ethnic violence. The Patrician, Lord
Vetinari, has responded by insisting that the Night Watch take on some
new recruits: a troll, a dwarf, and beautiful woman who’s also–but
that’s getting ahead of things.

One of the old writers of hardboiled detective novels–Raymond Chandler,
or maybe Dashiell Hammett–once said that when your plot is stuck, introduce a
man with a gun. And that’s what Pratchett has done here. There’s
exactly one gun on the Disc, and it’s in the hands of a man who thinks
that Ankh-Morpork needs a king again. Will Carrot play along? Therein
lies the tale.