This is a gripping story, all the more so as one had to remain very vigilant to keep track of who was where on their own and on each other’s timeline. This is the first level of vigilance: the complexity and entanglement of timelines
It is allied with a second level of vigilance: deliberately withheld information and hyper-manipulative disinformation combine to maintain the suspense-filled intrigue despite the frequently repeated assertion that you can’t change history. Everyone seems to agree with this principle, and to take it as a guide for their actions, even to obey plans based on known future facts or to renounce any attempt to alter the known sequence of events.
Yet there are ambiguities that work to cast doubt on or to undercut this determinist plot: the all-powerful time authority in Geneva that seems to be manipulating everybody, a small number of “Anachronists”, who believe history is mutable and are actively trying to improve it, some time-travellers who never came back and disappeared without a trace, “holes” in history both small and enormous, self-generating loops where a future self puts its past self on the pre-determined track, a mysterious 25th Century that doesn’t allow visitors and doesn’t travel, a set of prohibitions and protocols for time travellers that seem useless if history really is inviolable.
There is a time war being fought, and the reader begins to think that one side may win it by changing the past in its favour. Crucial items of information crop up late in the story and we wonder whether they were withheld by the authorities until then or only became true after active interference. This is the third level of vigilance: keeping track of the possibilities.
The stated goal of the protocols is to maintain not the timeline (which is held to be fixed) but the sense of agency. The theme of the book, however, is rather the degree of vigilance required of the characters, and also of the reader.