Guest stars:

None

Crew Appearances:
Kirk, Spock, McCoy
Chekov (navigator)
Uhura
Lt. Kyle (helmsman)
Nurse chapel
Sulu???? or a good look alike (in briefing room meeting)

Kirk and his crew are sent to investigate when an entire solar system is destroyed by a gigantic single-celled creature cutting a destructive swath through the universe. Kirk, Spock and McCoy are frantically trying to devise some means of stopping this entity when Spock makes a startling discovery - the giant amoeba is about to reproduce. They send Spock in and he's almost lost.
 
Star Base 6: Destination before sent to investigate the entity (for rest)

Sector 3-9J: location of the destructive entity

Gamma 7A: system in Sector 3-9J inhabited by 'billions', 'dead' from attack by the entity

Probes and results:
first probe: into the zone of darkness, casues half the crew to faint
second probe: into the amoeba, good data
third probe: into the nucleus to destroy it with anti matter

Shuttle craft NCC 1701-7 sent with Mr. Spock aboard to get further data.

Quotes:
"Are you trying to be funny, Mr. Spock?" Kirk
"It would never occur to me." Spock.
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"Astonishment" (what the crew of the Intrepid felt as they were being engulfed by the 'entity'.)
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"I cannot say what it is but I would say it has found us" Spock.
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"You have a martyr complex, doctor. I suggest it disqualifies you." Spock.
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"Good Luck, Spock." McCoy.
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"Antibodies. ANTIBODIES!" Kirk.