It is time to follow up in this marathon.
Time-Travel
Star Trek: Voyager
Relativity
Seven is recruited from the future to prevent the destruction of Voyager in the past.
I always liked episodes which plays with past occurances in the series. Here we got a little taste of it. They use a similar device (time rifts) in a future episode to have multiple instances of Voyager's past scattered throughout the ship ("Shattered").
A question I asked myself during this episode: Why should it not be possible to remove a person multiple times from their timeline? They say they have done it with Seven the third time now and probably further extractions won't be possible. But why? After a failed attempt, they always got a Seven who existed before the last extraction. Therefore for that Seven, it is the first time.
I liked the reference to Star Trek: First Contact. The acknowledgment of the causal loop generated by the time-travelling borg and thus the arrival of the Enterprise making the first warp flight possible when they assisted Cocraine.
By the way: Seven looks great in a Starfleet uniform. A shame they did not make it her regular outfit.