Have you ever overheard someone talking about a book you haven't read yet, and they give away a major plot element? Then when you read it, you spend most of the time wondering how the author is going to treat that twist, how the death of a central character or a personality change is going to affect the narrative.
And then there's the times when the person you overheard was ON CRACK, and said plot element never happens, so you get to the end and are both glad that it never happened and royally ticked because someone still managed to ruin the read for you by making you dread the moment.
Herm. Anyway. As the title suggests, this book is part of the Discworld cycle dealing with the Night Watch, and, more particularly, Captain Vimes and his idea of what it means to be a keeper of the peace. Vimes is a keen study of human nature, and though he thinks of himself in less than glowing terms, he has a finely developed sense of morality, and, more importantly, a sense of what is right. When chasing down an unrepentant murderer (utterly classic sociopath, and a charismatic one), a thaumic incident sends them both back in time, where the murderer kills John Keel, a very important personage in terms of his effect on Vimes' life. In order for there to be a future for Vimes to go back to, he has to assume Keel's role... in part, so that he doesn't turn out to be someone he doesn't want to be.
This novel is shot through with examinations of responsibililty. Vimes has to be true to himself, which means doing the right thing even when the wrong thing would be far more expedient and possibly smarter. Of course, he's also having to provide an example to his younger self, and he doesn't dare provide a bad one; there's already far too many of those around.
As with most of Pratchett's work, there are plenty of good lessons in character slipped into the narrative. In this particular book, the comedy is perhaps darker than in previous novels, but the people therein are worth emulating. Except for Nobby Nobbs. Don't even think about emulating him.