The past is a different country, something that Sam Vimes finds out the hard way. During a conflict with a murderer Vimes and Carcer, the murderer, get transported three decades into the past. The city is controlled by an insane lord, whose secret police is torturing and killing innocent people. And the Night Watch, the organization Vimes will lead in the future, is just a shadow of what it will be, full of corrupt policemen. The smart choice would have been to sit one the sideline, watching history unfold. But Vimes has to take action to preserve his future. The man who taught him everything when he was just a young copper, was killed by Carcer, and Vimes has to take his role now.
If you liked the first Night Watch books, where the plot still concentrated entirely on Ankh-Morpork and its politics, and the Watch was still very small, then you’ll like this one. It was a smart move by Pratchett to put Vimes back into a setting were he couldn’t rely on all the power he amassed over the time. He is back to square one, and the only thing that he has is his skill and knowledge.