Where it should have been: #19, so a solid episode all around.
In case you forgot: The Enterprise is investigating some mass madness weirdness going on in a solar system. The planet Deneva, where Kirk has family, is next. Turns out the planet has succumbed to the madness as well—it's brought on by little puddles of fake vomit that latch on to people's backs. Fortunately, it turns out that UV light can kill it. (Somehow this planet doesn't ever get UV light? Atmospheric anomaly? I don't know.)
The good: Even if the monsters do look like goofy puddles of fake vomit, the threat they represent is a pretty frightening one. "Operation: Annihilate!" is a nice variation on the typical body snatchers theme, in that the entities doing the snatching are just thoughtless members of a giant collective enemy.
The bad: Spock going blind but then not really is a bit of unnecessary tacked-on drama. The creatures' aversion to UV light is also a cop-out solution on par with the aliens and water in War of the Worlds: it's nice that the day is saved, but it feels kind of unsatisfying.