The Discworld and its denizens keep moving forward.
In particular, the much-reviled police captain Vimes and the much-honored Duke Vimes move forward. I mean, like a glacier moves forward. Not the fastest one around, I won't even warn you to get out of his way. Glacier-like, it wouldn't matter. Go ahead, get in his way - he might even notice. Probably not.
This time, in his ducal capacity, he has been appointed to an ambassadorship by Lord Vetinari. Vetinari is not a bad man (by local standards, at least) and doesn't do bad things (again, by local standards). Pray that you're nowhere near when he attempts something good. It might be like lighting a candle in the darkness, with you as the match.
Or it might be like lighting the fuze on the powder-keg. Vimes isn't much the candle type. Around him are many people. There's his finishing-school wife who can finish off dwarves and lots of others, six against one, in unarmed debate. There's Officer Angua of the city watch. A very capable woman but watch out for her "monthlies". You know, new moon, howling over the heath, and and all that were-sort-of-thing. Then ... well, Angua is the predictable one. There are lots of others who aren't.
This is a long-running series with lots of character development in previous volumes. Pratchett is uncommonly well tuned to the newcomer, though. Even if the writer knows the two-dozen stories before this one (and a dozen-squared he never wrote), this story still stands well on its own. The newcomer may as well start here as anywhere. The tone is a bit more serious and less haha than most of the Discworld series, but it fits well.