"Obsession" by Art Wallace
Vampire cloud monster! I know I haven't seen this one very often. Kirk goes Ahab on a creature he encountered 11 years earlier that killed half the crew of his ship, the
Farragut, including his captain.
It starts with a familiar scent. Redshirts die, drained of hemoglobin. Kirk takes more redshirts, including Ensign Garrovick, who happens to be the son of the Farragut's captain. Garrovick hesitates in firing on the creature, just as Kirk did back when he was a young officer. Kirk's guilt drives him to chase the creature when it leaves the planet, pushing the Enterprise until it nearly explodes. McCoy and Spock are worried about him and confront him over his obsession. The chase is delaying a rendezvous with the Yorktown to deliver medicine.
The creature shows signs of intelligence, even turning on the Enterprise and getting into the ventilation. It gets into Garrovick's quarters while Spock is there. Spock throws the ensign out of the room and then tries to shut the vent, which Garrovick accidentally broke (he also tries to stop a cloud with his hands, which is just stupid). Scotty reverses the ventilation and Spock comes out unharmed, as his hemoglobin is based on copper, not iron. Since none of their weapons have effected the creature, Kirk lets Garrovick know he has no reason to feel guilty - and realizes *he* has no reason to either.
Kirk has some kind of link to the creature and when it moves off, he realizes it's going home, to where it met up with the Farragut 11 years earlier. Spock thinks it's going there to spawn. Kirk and Garrovick beam down with hemoplasm to attract the creature and an anti-matter bomb to take it out. The creature gets the bait before they can set up, so Kirk decides to use himself. Garrovick fights him, trying to take his place, but Kirk says he doesn't intend to sacrifice himself. They lure the cloud to the bomb and... "Energise and detonate!"
Spock has to pull some rabbits out of his hat, but manages to beam the two aboard. I think McCoy's issues with transporters start here ("Crazy way to travel, spreading a man's molecules all over the universe.").
A tight, suspenseful episode. Shatner does a terrific job showing Kirk obsessed, guilt-ridden, and also questioning whether he's doing the right thing. Stephen Brooks does a good job as the young Garrovick. Chapel has a great scene where she's trying to get Garrovick to eat.
Amusing bit: "'Lieutenant Lesley' played by regular Star Trek background actor
Eddie Paskey is killed in this episode, however his character re-appears (and is referred to by name) in many subsequent episodes."