Kirk leads an expedition to find tritanium, a diamond-hard ore valuable to the Federation. They discover it on the uninhabited planet but the captain notices a honey-like odor that he seems to remember. He orders Ensign Rizzo to take a team to search for a cloud and scan for di-kironium while keeping an eye out for a gaseous cloud. As he sends the team out, Scotty calls to remind him that they are due to rendezvous with the Yorktown
in eight hours to receive and deliver vital medical supplies for Theta 7. Rizzo and his two men pick up traces of di-kironium but are unable to get a fix. A cloud sweeps over Rizzo's men, killing them, and then closes in on Rizzo. He signals to Kirk, who comes running with the others only to find Rizzo in critical condition and the other two men dead, every red corpuscle drained from their bodies...Read the full recap
Spock: Pure tritanium.
Kirk: Fantastic. 20 times as hard as diamond.
Spock: 21.4 times as hard, to be exact.
Spock: I hope I'm not disturbing you, Doctor.
McCoy: Interrupting another autopsy report is no disturbance, it's a relief.
Spock: I need your advice.
McCoy: Then I need a drink.
Spock: I do not understand your reasoning.
McCoy: You need advice from me? You must be kidding.
Spock: I do not joke, Doctor. Perhaps I should rephrase my statement. I require an opinion.
Kirk: Maintain search. Kirk out. It can't have just vanished.
McCoy: Sometimes they do... if we're lucky. Monsters come in many forms. You know the greatest monster of them all, Jim? Guilt.
Ensign Garrovick: What's happening?
Chapel: Are we still chasing that thing half way across the galaxy? Yes. Has the captain lost his sense of balance? Maybe. Is the entire crew about ready to explode? Positively. You're lucky you're out of it.
Chapel: You know, self-pity's a terrible first course. Why don't you try the soup instead?
Spock: May I suggest that we no longer belabor the question of whether or not we should have gone after the creature? That's academic. The creature is now after us.
Spock: The fault was not yours, Jim. In fact, there was no fault.
Kirk: If you want to play analyst, Spock, use someone else, not me. My concern is with the ship and the crew.
Kirk: Don't misunderstand my next question. Mr. Spock, why aren't you dead?
McCoy: It's that green blood of his.
Spock: My hemoglobin is based on copper, not iron.
McCoy: I'll bet he left a bad taste in the creature's mouth, too.
Spock: Colloquially expressed, but essentially correct.
Scotty: Captain! Thank heaven!
Spock: Mr. Scott, there was no deity involved, it was my cross circuiting to B that recovered them.
McCoy: Well, then, thank pitchforks and pointed ears!
Lt. Lesley (Eddie Paskey) dies in this episode, one of the first two victims of the cloud creature. However, he is seen later in the episode outside of Sickbay, wearing a yellow tunic. And the character will appear subsequently in many future episodes.
When Kirk calls for beam up and detonation, he's holding his communicator in his hand and is crouched over. When he eventually materializes on the ship, he's standing up and the communicator is gone.
McCoy is seen wearing an engineering badge on his medical tunic in several scenes.