“Force of Nature” (7×09)
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This should have been a game changer: Warp travel is inherently destructive, with the effects accumulating over time. The Federation is so shaken by this that it immediately restricts travel to Warp 5, essentially turning back the clock to the “Enterprise” days. And over time, the warp corridors would get smaller and smaller, making interstellar travel far more fraught and complicated to navigate.
The timing of this episode was strategic: The only other Trek show was set on a stationary space station, and most travel happened via a wormhole rather than via warp. So it wouldn’t have to box them in too much dramatically. And it could have even been a story engine: Perhaps Voyager could have found the technology from a non-destructive means of warp travel from a civilization in the distant reaches of the Delta Quadrant. Then there would have been mission-critical stakes in addition to the personal ones: They needed to get back not just for themselves, but to solve this huge problem the Federation has.
But aside from a couple references to the Warp 5 speed limit in later Season 7 episodes, as far as I know the danger was never addressed again. If you’re going to make an environmental allegory about the need for short-term sacrifices in service of the long-term greater good, I think you have a responsibility to actually show those short-term sacrifices.
The episodic format of TNG actively works against such big swings to the canon having lasting impact. Their instincts were not to carry forward consequences into subsequent stories, but to look at them as more or less a clean slate.
I also think there’s way too much time spent on a pointless subplot about Data’s cat.