The Enterprise ventures into a corridor of space in search of a missing Starfleet vessel but instead finds a disabled Ferengi ship, then later a debris field that's actually a trap used to disable the Enterprise. It was set by the inhabitants of a nearby planet used to raise awareness; two of their scientists, Rabal (Michael Corbett) and Serova (Margaret Reed), claim the cumulative use of warp drive in this narrow corridor is causing irreversible damage to their space and planet. Also, there's a lot of dialogue about Data's cat.
Structurally, "Force of Nature" is about as inelegant as it gets. For the first half of the episode we essentially have a B-story where Data and Geordi crawl around Jeffries tubes and talk about Spot the Goddamn Cat. This material is so clearly padded (and then abandoned) that I'm pretty convinced the writers realized they didn't have enough story to fill the hour and so they tried to turn what should've been the teaser (and absolutely nothing more) into 10 minutes of irrelevant subplot. Then we get the disabled Ferengi ship, which is ham-handed to say the least (the Ferengi captain is a hostile idiot who makes threats and wild accusations despite any evidence and his inferior position that's completely at Picard's mercy) — and again, is dropped so soon after it's introduced that it feels like filler.
Finally we get the meat of the story with the alien scientists, who are desperate to prove theories that have been skeptically received for years, which is that warp drive is damaging their space. When it doesn't look like Serova is getting the immediate response she wants from the Federation, she pilots her ship into the rift and breaches the warp core, killing herself in a radical demonstration that opens a spatial rift that (sort of) proves she was correct. Serova does this despite the fact that Picard was fully prepared to take her recommendation to Starfleet, whom we have no reason to believe won't take it seriously. Serova is, in short, the story's dramatic catalyst, but not a particularly believable one. There are some okay scenes in the wake of this, including Geordi navel-gazing and his conversations with the reasonable Rabal, brother of the late, radical Serova.
The last act, in which the Enterprise must enter and escape the rift to rescue the survivors of the missing ship, drowns in a morass of who-cares technobabble. It makes you realize just how bored this kind of rote recitation of jargon must make the actors. It's action of the most thankless kind, where the camera shakes while people urgently yell out jargony jargon words, and it's supposed to be exciting, but it really is not. Better, but not good, is the heavy-handed final scene in the conference room, where we get the fallout from the revelation that warp drive might actually be causing damage to some areas of space. (The story is sketchy on the details, so as to not lock Star Trek into a situation where one can't trek the stars.)
This is a Trekkian Message Show with themes delivered with the subtlety of a sledgehammer. The real-world corollaries might register as topically relevant if they weren't so laughably obvious in their delivery. To summarize: warp travel is equivalent to carbon emissions, which is causing damage to space, with the long-term devastating environmental effects being equivalent to global warming. I will say that this is not one of Trek's more enlightening or effective allegories. (The Federation's immediate short-term precautionary solution is to institute a speed limit of warp 5, which comes across as more silly than anything.) The one irony I will note, writing this 19 years after "Force of Nature" aired, is the sad state of where the issue of global warming has since ventured in America — with accepted science being pathetically reframed as debate by absurd and shortsighted economic/political denial.
Come to think of it, the fact that the Federation enacts a speed limit immediately after the events of this episode says something when you consider what we haven't done to address carbon emissions and other ecological problems in the 19 years since this episode aired. Perhaps this episode is the ultimate allegory of indictment, meant to be analyzed decades after it was made! Now there's a concept!
Okay, or not.