In a long-running series there is often a problem with escalation. Characters that started in a low station move up in the world. They face new challenges as they rise; the stakes are implicitly higher for the fictional world, as their actions can now affect more people. At some point, though, the character rises as high as the author is willing to allow, and character development has to come about in another fashion. Pratchett has already faced this set of questions with Granny Weatherwax. She has grown in power from Equal Rites to Carpe Jugulum, and challenges to her are now no longer likely to come from the outside. The Sam Vimes of Guards! Guards! — drunkard captain of the disreputable Night Watch — has become the Duke of Ankh, Sir Samuel Vimes, Commander of the Night Watch, an organization that has become a model of effective policing, so effective that coppers from Ankh-Morpork are in demand across the Disc and are commonly known as Sammies.
As Night Watch opens, Vimes is caught up in management of the growing Watch, in negotiations with the city’s leadership about budgets and powers, and in general wishing for simpler days when he was out walking the streets and either chasing criminals or being chased by them. Vimes has never been one for politics, and has always relished what he sees as straightforward police work, so this development is not necessarily new. On the other hand, it’s also easy to see an author looking back fondly on simpler situations and more straightforward action. In a setting teeming with magic, it’s not difficult for the character’s yearnings to be fulfilled, if not quite in the way he expects.
A sorcerous conflagration blasts Vimes thirty years backwards in time. A ruthless killer goes back with him. Both survive an initial attempt at arrest and murder, and make their separate ways into the city. Vimes joins the Watch, taking the identity of a new Watchman who was killed in the tussle that accompanied Vimes’ time travel. Carcer, the killer, joins the Unmentionables, the secret political police that operated in Ankh-Morpork at that time, seizing the apparent power of law to support his own vendettas.