This is the 27th Discworld novel (well, that is, if you don't count The Last Hero and The Amazing Maurice and His Educated Rodents).
It is springtime in Ankh-Morpork, the lilac is in bloom. As his wife Sybil is about to give birth to their first child, Commander Samuel Vimes of the City Watch heads to the cemetary of Small Gods, to commemorate the day Sergeant John Keel, his mentor, and six other coppers died some thirty yeas ago.
Later, arriving at the Patrician's Palace, he hears that Carcer, a serial killer who's been wreaking havoc around town lately, has just been cornered. This might be his only chance to arrest the murderer.
Outside, there's a storm brewing. After a chase in the streets of the city, Vimes and Carcer end up in the tower of the wizards' University, a highly magical place. And as the Commander is about to catch his prey, lighting strikes, and both are transported back in time, some thirty years earlier... Soon Carcer commits another crime and kills John Keel.
Night Watch has a strong "Back to the Future" theme, where changing events in the past... well, the now, of course affects those in the now... well, the future. Many things have changed in thirty years, and Vimes struggles to put his own past back on the track. It won't be long until he encounters his younger self. Passing himself off as Sergeant John Keel, not only will he have to teach young Sam to be a good copper, but he must also survive the oncoming Revolution.
True to form, Terry Pratchett gives us yet another witty, intelligent, hilarious Discworld novel of the City Watch, with its traditional footnotes and tongue-in-cheek humour, and some cameo appearances of Death... what more could we possibly ask for?