The opening credits are a warning to fans: "Inspired by the works of Terry Pratchett." My emphasis. Not "based on." They mix a semi-modern city with Pratchett's later re-envisionings of technology, and mix in the happenings of a couple novels about "The Watch" in the city of Ankh-Morpork. This includes Captain Sam Vimes (Richard Dormer), Constable Carrot (Adam Hugill), Corporal Angua (Marama Corlett), Corporal Cheery (Jo Eaton-Kent), Sergeant Detritus (Ralph Ineson), and Lady Sybil Ramkin (Lara Rossi).
The primary plotline has Vimes's former best friend and criminal nemesis Carcer Dun (Sam Adewunmi) somehow resurrected from the dead, and attempting to wipe out the entire city by controlling a huge dragon to burn it as some form of revenge. Vimes has been driven to drink by Lord Vetinari (Anna Chancellor) legalizing many forms of crime and the resulting inability of the Watch to do anything about most crimes ... but now Vetinari expects him to do something.
The list of problems as a fan of Pratchett's novels is immense. There are some good things: Eaton-Kent as Cheery was an inspired choice, Corlett is good as Angua, and Dormer as Vimes is quite good - I'm tempted to say "over the top," but that's how Pratchett painted the character. Carrot was a very important character in the original stories, but here he's ... determined but almost non-descript. That was a hard row to hoe though - Carrot would be particularly hard to bring to screen successfully. And I was unhappy with Chancellor as Lord Vetinari: ever since Charles Dance played the role in "Going Postal," I've had an extremely clear image of the character in my mind, and it's Charles Dance. He was perfect. The inclusion of Lady Sybil in the Watch is problematic, because in the stories I don't think Lady Sybil was ever a part of the Watch. She was definitely associated with them indirectly because of her involvement with Vimes, but here she's portrayed as a axe-wielding badass.
The single biggest problem is the dilution of Pratchett's humour. They've significantly re-arranged the plot, so they can't use many of the original jokes. And the jokes that they've put in to replace them aren't as funny, and there's just not enough Pratchett left to keep the motor running. I was mostly with them right up until the musical number - not when the Watch tried out for the Musician's Guild (I survived that, barely), but the actual musical number in around the fifth episode with singing and dancing. I stumbled through the rest of the episodes out of a sense of duty, to get to the unsatisfactory ending that includes setting up a new big-bad for the next season they probably won't get.