There was a great Sci-Fi horror movie back in the 1987 called The Hidden (and Hidden II), about a parasite that takes over humans and makes them stronger. This episode is almost a sequel to those two movies about aliens that wanted to invade earth.
Of course, there are the great classics like The Thing from Another World (1951), Invasion of the Body Snatchers (1956), and The Puppet Masters (1994, with Donald Sutherland, who was also in the 1978 remake of Invasion of the Body Snatchers).
Unlike a lot of the ST:NG episodes that were all about the high-minded Federation and its Prime Directive, this one is just straight up classic Sci-Fi horror. Nasty parasitic bugs are getting into the top leadership of the Federation, and their goal is to control all of the starship commanders and all of the Federation admirals.
They seek to become the Bug-Lords of the universe, and they are getting access to Federation leaders thanks to Dexter Remick, the nasty guy that was trying to get Picard court-martialed back in episode 18 (Coming of Age). Apparently his creepy persona was also spreading around space parasites. Picard dodged a bullet the first time, but Remmick and his crew of parasite-infected admirals is out to get him now.
Thanks to Worf, Riker, Geordi, Dr. Crusher, the good guys eventually figure out what's going on. The parasites at the end of this episode look like a bag of Party City scary critters was being pulled by strings. That was a bit disappointing, but the master slug parasite inside Remmick was pretty gross.
Overall, this is a fun episode that I enjoy watching many times, unlike some of the episodes about Federation dogma and philosophical questions regarding the Prime Directive that gets violated more often than all the Kardashians at an NBA basketball camp.