Warning: Wikipedia contains spoilers
On the morning of the 30th anniversary of the death of Vimes' hero - John Keel, the copper who taught him all he knows about being a copper - Vimes is caught in a magical explosion while pursuing a serial killer across the roof of Unseen University. Both Vimes and the escaping criminal are thrown 30 years back in time, where the criminal murders John Keel and Vimes is forced to become the copper who taught him all he knows...
Nightwatch is also the name of a fictional paramilitary intelligence organization in the science fiction drama Babylon 5.