Where it should have been: #31, so another decent-to-good workhorse of an episode.
In case you forgot: Kirk becomes obsessed with destroying a cloud-like monster that he's convinced it's the same creature he encountered eleven years ago on the USS Farragut. He gets pretty close to going full-on Ahab, but his obsession pays off and they're able to kill the monster.
The good: The monster and what it does—draining all the red blood corpuscles from someone's body—is pretty cool. I always like the monsters that are just these giant thoughtless blobs of instincts and reactions. Not all life has to be intelligent, after all. This episode is also as close as it gets to Kirk really screwing the pooch, which is a nice foil to his usual hypercompetent self.
The bad: The episode seems to undercut its whole point by making Kirk's obsession pay off (by way of a dead monster). I really wish the danger would have come not from the monster of the week, but from Kirk being wrongly obsessed, and that the issue had been resolved by Kirk coming to his senses, not killing another monster.