One of the tricky parts of a time travel novel is explaining to the traveler (especially if his journey is inadvertent as it is in "Night Watch") and the reader just what the heck happened.

Luckily, in a previous Discworld novel, "Thief of Time" (2001), Pratchett invented Lu-Tze and the Time Monks, so that when Sam Vimes travels back to his own past while chasing a cold-blooded murderer, a little bald wrinkly smiling man (Rule #1: "Do not act incautiously when confronting little bald wrinkly smiling men!") rescues Sam from the clutches of history's cowardly, badly run Night Watch, and explains what has happened.

Basically, young Sam Vimes has just joined the past's Night Watch and the older Sam Vimes must teach him to be a good cop while disguised as Sargeant John Keel, newly arrived in Ankh-Morpork and ready to kick some sloppy, sadistic cop butt.

If 'old' Vimes doesn't make sure 'young' Vimes becomes a decent cop, his future, his wife, his position as Commander of the Watch will be lost to him. Good men will needlessly die.

Historical Ankh-Morpork is also gearing up for a revolt against a Patrician who is propped up by a really nasty secret police gang. Once 'Sargeant John Keel' gets himself clothed, out of jail, his plight explained by Lu-Tze, and is put in charge of the Treacle Mine Road Watch House, he must train up young Sam Vimes (as well as future-Sargeant Colon, and Nobby Nobbs), capture a killer, and make sure the Revolution is won by the right people.

Only then Vimes will get to return to the 'future' where his wife is about to give birth.

I enjoy all of Discworld novels starring the redoubtable Sam Vimes, but "Night Watch" is something special. The plot zips from beginning to end, never slowing down except for the mandatory insertion of the Time Monks, who have to explain the how and why of Sam's excursion into his own past. Many of the loose ends from previous Night Watch novels are tied together and neatly tucked in. Most of all, I was delighted by a deepening illumination of character--not just of Vimes, but of the Patrician Vetinari, Sargeant Colon, and Nobby Nobbs. While you read, you'll be thinking 'of course they started like this.' Pratchett is that good.

If you'd like to read the Sam Vimes/Night Watch books in order of publication, they are: "Guards! Guards!" (1989); "Men at Arms" (1993); "Feet of Clay" (1996); "Jingo" (1997); "Night Watch" (2002); and "Thud!" (2005).