On the morning of the 30th anniversary of the Glorious Twenty-Fifth of May (and as such the anniversary of the death of John Keel, Vimes' hero and former mentor), Vimes is caught in a magical storm while pursuing Carcer Dun, a notorious serial murderer. He awakens to find that he has been rescued by Miss Palm (whom Vimes knows as Mrs Palm, Head of the Guild of Seamstresses). He determines that he has somehow been sent back in time.  Vimes attempts to employ the wizards at Unseen University to send him home, but is arrested at arrow-point for breaking curfew by a younger version of himself. Incarcerated in the cell beside his he finds Carcer, who after being released joins the Unmentionables, a secret police carrying out the paranoid whims of the Patrician of the time, Lord Winder.  When he is taken up to be interrogated by the captain, time is frozen by Lu-Tze and he tells Vimes what has happened and that he must assume the identity of his mentor Sergeant John Keel (who was to have arrived that day but was murdered by Carcer). It is stated that the event which caused Vimes and Carcer to be sent into the past was a major temporal shattering. Possibly, the cause of Vimes going through time is that he was caught in the storm at the same instant the glass clock struck and time froze in Thief of Time. This implication is further underlined by the mention previously that the bolt of lightning which triggered the magical explosion hit a clockmakers shop, stopping all the clocks within. Vimes then returns to the office, time restarts and he successfully convinces the captain that he is Keel.  Young Vimes believes Vimes to be the real Keel, allowing Vimes to teach Young Vimes the lessons for which Vimes idolised Keel. Essentially this means that Vimes taught and idolised himself, not Keel, although alternate histories and the "Trousers of Time" mean this may not be the case ("You were indeed taken under the wing of one John Keel, a watchman from Pseudopolis," says Lu Tze. "He was a real person. He was not you").  The novel climaxes in the Revolution, hinted at since the start of the book. Vimes, taking command of the watchmen in his troop, successfully avoids the major bloodshed erupting all over the city and manages to keep his part of it relatively peaceful. After dealing with the Unmentionables' headquarters he has his haphazard forces barricade a few streets to keep people safe from the fighting between rebels and soldiers. However, the barricades are gradually pushed forward during the night to encompass the surrounding streets until Vimes finds himself in control of a significant part of the city of Ankh-Morpork.  The ruler, Lord Winder, is assassinated (by Havelock Vetinari) and the new Patrician Lord Snapcase calls for a complete amnesty. However, he sees 'Keel' as a threat and sends Carcer and the palace guard to murder the Night Watch. Several policemen are killed in the battle (both in accordance with the "history" and explaining why they are not seen in other books); Vimes manages to fight off the attack (with his policeman and with Vetinari helping) until he can directly grab Carcer, at which point they are returned to the future and Keel's body is placed in the timeline Vimes' has just left, to tie things up, as in the "real" history, Keel died in that fight.  Vimes' son is born, with the help of Doctor 'Mossy' Lawn (who Vimes met while in the past), and Vimes finally arrests Carcer, choosing justice over his strong desire to kill him. It is then revealed that Patrician Vetinari realised that Vimes was 'Keel', since the body of Keel looked rather different to Vimes when he was alive and because Vetinari meets Vimes immediately after he has returned in time (and so is looking exactly like Keel from the time of the fight).

A very good story that's part Terminator and part Terminator 2, with a bit of 12 Monkeys thrown in for good measure. I liked the scenes with Vetinari, very cool and we get a sense of just how he came to power and why he's been able to survive in office. The scenes with the Agony Aunts were fun too, as was seeing a very young Nobby Nobbs erstwhile of the Beggers Guild. It was almost a shame that Vimes returned to the 'present day' at the end as I'd have liked to have read more about Keel's times, or at least what happens after his death, we know Snapcase turns into a total loony but so little of his era is recorded...