A fairly late book in the Watch series. Improves on being read in sequence
The life of Sam Vimes. Living the uncomfortably posh life -- he can't really go on patrol, because the sergeant alert the men to straighten up, and the consolation of knowing he has sharp sergeants does not compensate for the lack -- and his wife is in labor -- and he gets word that a homicidal sociopath, Carcer, whom they've been looking for, had run into a guardsman just off duty and killed him. Top priority comes to cornering him, which ends up on top of the Unseen University When Vimes goes to bring him, lightning strikes.
Next thing he knows, he's lying in the street. It takes him some time to work out that he's been thrown back in time. His wife is still a girl. His younger self is new to the Watch but quick-witted enough to arrest him. Lord Winders is Patriarch and paranoid, and his Unmentionables are doing his (very) dirty work.
The History Monks find him and tell him that Carcer has set time awry. They can sustain things if he moves into the shoes of a person whom Carcer kills, significant in his past.
It involves inkwells, lilac blossoms, Dr. Lawn an irregular doctor, needing a receipt to hand over prisoners, a silver cigar case, the inconspicuousness of dancing through the street lighting firecrackers, where the second-hand store got its used clothes, setting a building on fire, and much more.