4 ideas
Jud Elliott III, a bored young man, becomes a Time Service courier leading tourists through Byzantine Constantinople. He tracks down his own many-times-great-grandmother, Pulcheria Ducas, falls in love with her and sleeps with her.
If tourists from the future can visit famous historical moments, every future sightseeing trip adds more observers to the same event. Over time, landmark scenes such as the Crucifixion fill with disguised time travelers who outnumber the original participants. The historical record then documents crowds that were created by later spectators.
The Time Patrol does not imprison people who break the rules. It goes back and removes their existence, stopping the conception that would have produced them. Because the offender never existed, their crime and their identity disappear together. The state's power over time becomes power over whether a person ever lived.
When history can be visited, it is packaged as consumer entertainment. Tourists and couriers treat plagues, riots, coronations and even their own ancestors as attractions or chances for sex, with no sense of obligation. The novel's satire shows this detachment is only sustainable while someone else pays the cost. The comedy turns grim once the tourist can no longer walk away from what he disturbed.