Evolution exists or it doesn't to some people and if you showed this episode to a non-believer it will certainly ENSURE they won't change sides and could entertain them.  Some people find this a cracktastic blast or insultingly stupid.

In the episode, Data and Picard leave the ship to retrieve a stray photon torpedo leaving Riker in charge of the Enterprise while other things are going on.  In the teaser, we see Crusher curing Barclay of a disease that releases a virus throughout the ship.  Riker begins turning into a caveman, Worf a primeval Klingon, Ogawa a wolf and O'Brien is a leprechaun.  No just kidding.  He wasn't on the show anymore.  ha ha ha.

Returning to the ship, Data and Picard find all manner of things wrong, the power out and dead crewmen.  Data needs to find a cure for what has imparied the crew.

There is no real villain here other then the virus.  There are some action beats in the second half where the episode turns into a survival horror episode with thumps in the dark and weird creatures.  How much you like these scenes depends mostly on how much you like the episode since all the science in the show is garbage and the story itself isn't anything special.  This episode for me was just fun to watch and enjoyable as hell, before things unfold and after.

This makes a lot of "hate it" lists for Trek but it isn't that bad.  The only thing you have to lose is 40 minutes of your time if it stinks.  If you have any kind of Science degree, the ink will run off the diploma as you watch.