This cheesy Trek may be repulsively silly at first, but reveals a surprisingly sensitive, character-based climax.
The episode suffers from its conflict-of-the-week being almost entirely disparate from its dramatic heart. This creates a very unbalanced narrative structure that pushes all of the meat to the last half of act three. The majority of the episode concerns itself with the mystery of the flying-scabs-that-latch-onto-people's-backs-and-make-them-crazy.
The scabs aren't really scabs, of course, but enlarged single cells of a greater organism from another galaxy. They're going from system to system, using the bodies of unsuspecting humans to do their bidding or perish. Mr. Spock is attacked and is only able to fight back through use of his advanced Vulcan mental discipline. This character struggle seems misused for the visceral display of Spock's attempt to take over the bridge rather than a legitimate opportunity to display Leonard Nimoy's acting ability.
The use of Kirk's brother Sam as cannon fodder for these creatures in the opening of the episode seems forced. Even worse, it's used to surprisingly little effect. While Kirk may initially voice concern for the solving of this issue to save his nephew, he quickly acknowledges the larger implication of saving the entire planet below. The dramatic struggle for his character fizzles with little effect on the story proper.
The depiction of the Kirk-Spock-McCoy relationship towards the end of the episode is spot on but brief. Reduced to a single conference room sequence, we see Spock and McCoy present two entirely unsatisfying options to Kirk as to how the situation might be remedies. Kirk refuses to accept either and demands a third option. Simple, brief, I wish there was more of it. It was the only drama that was really founded in the story this episode contained.
The other standout piece of conflict belongs to McCoy, and has no intrinsic relation to the story. It could have been placed in any episode. When testing ways to defeat the creature, McCoy uses Spock as a test subject before preliminary results had been viewed. This oversight causes Spock to be blinded. The way in which his guilt is depicted dramatically through the final scenes is so sensitively done it betrays the sloppiness the permeates the entire episode leading up to it.
Spock's blindness was temporary (No!), ending the episode on a sweet note between them on the bridge. One of the few times McCoy lets slip a compliment to the Vulcan first officer is a delight to behold.
"Please don't tell Spock I said he was the fleet's best first officer."
"Why, thank you, Dr. McCoy."
"You've been so concerned about his Vulcan eyes, Doctor, you forgot about his Vulcan ears."