There's a saying in screenwriting that goes something like, "There are no bad plots, only bad executions." I beg to differ. "Genesis" is a bad plot. It's also a bad execution, but I hold that nothing could have saved a story so fundamentally flawed.
Of course, that all depends on what you can abide in your definition of Star Trek as science fiction. I can abide plenty of impossible Trekkian staples, like warp speed, time travel, or the transporter. What I cannot abide is Fun With DNA™ when it goes as far as "Genesis" does (or the even more unwatchable "Threshold" on Voyager, to which "Genesis" seems to have served as Brannon Braga's warm-up act). "Genesis" tells the story of how the entire crew is inflicted with a condition that starts rewriting their DNA and "de-evolving" them into more primitive forms of life. Riker becomes a Neanderthal. Barclay becomes some sortuva spider/humanoid thingy. Troi grows gills and turns into a half-woman, half-amphibian. Worf turns into a violent monster that terrorizes the ship. And so on.
The thing about Fun With DNA is that it can do anything the plot wants, so nothing that happens means anything, because it can all be undone with the wave of a hand: You don't need to see his identification. The concept of "de-evolving" (was the word "devolving" deemed too far over the audience's head?) is so silly that I rejected it outright. But then the way everything is fixed with a magical potion just makes you want to wash your hands of the entire episode, since clearly the writers were happy to do the same. In any plausible notion of this story, the changes to your DNA should simply kill you. In "Genesis" it allows the writers to explore their inner monster movie. A bad monster movie.
The first act, when the crew starts to go nuts, is actually kind of entertaining in its zaniness. Everyone starts acting strangely and bouncing off the walls. Riker turns suddenly stupid. Troi takes a bath with her clothes on. Barclay is weird — okay, weirder than usual. Worf is non-communicative and aggressive, and then sprays Crusher in the face with venom, which is actually kind of amusingly horrifying as she goes thrashing about excessively.
Picard and Data, who were away when this happened, return to find the ship is now a funhouse of mutations. This is where things turn tedious, as we find out who has changed into what and then must figure out a way to magically reverse the effects. (Naturally, there's no shortage of technobabble.) Picard, now being exposed, finds he too is "de-evolving," which for him has the initial side effect of making him a coward, which is admittedly sort of fun to see Patrick Stewart play. Meanwhile, Spot has transformed into a lizard. Why a lizard? Probably because it was easy to get a lizard and film it, and then claim it was Spot.
So, yes, this is obviously terrible. Much of the action centers on the fact that Worf is now a monster who believes Troi is his mate (hey, it's another example of Worf/Troi Will They or Won't They as filtered through unreal sci-fi machinations!) and then goes chasing Picard through the corridors after being lured away from doing unspeakable things to the helpless Troi-fish. Ah, the ignominy of being Klingon Guy Worf: When you get your DNA mutation episode, you're the first choice for the monster who eats the town.
When looking at the stretch of episodes from "Homeward" through "Genesis" ("Lower Decks" aside, of course), it's not hard to make a case for this cumulatively being the nadir of TNG. Granted, I'd rather watch this than season one, because the characters and production are more honed. But season seven should know better.