Agents can only travel to moments and places already documented by surviving historical records, because the records are what anchor and guide the jump. This makes the archive itself the infrastructure of movement: where no record exists, no travel is possible, so history's blind spots become physically unreachable.
A civilization that survives near-extinction and gains control over time does not transcend racial and bureaucratic hierarchy; it reconstitutes it, assigning value and suspicion along inherited lines. Mastery over history does not purify the society wielding it of its prejudices.
When causes can travel backward, a plot has no clean origin point: the investigator's own interventions may be the events that create the conspiracy he is chasing. Viewing threat this way dissolves the distinction between detective and culprit, since acting to prevent an outcome can be the mechanism that produces it.