Operation -- Annihilate!

In the episode, the crew of the Enterprise must find a way to exterminate behavior-altering parasites that have taken over the bodies of residents of a Federation colony, including Captain Kirk's brother, Sam, and family.

The USS Enterprise is tracking a path of mass insanity that has affected several planets, causing the collapse of their civilizations.

The party locates the Kirk family home; Sam (played by William Shatner) is dead, while Aurelan (Joan Swift) acts irrationally before passing out and Peter (Craig Hundley) is comatose.

They transport back to the Enterprise, and McCoy determines that the creature has injected some of its tissue into Spock's spinal column; it will be impossible to remove surgically.

Later, Spock apologizes to Kirk and asserts that his mental discipline can control the pain, but that he must return to the surface to acquire a specimen to study.

Spock then volunteers to be exposed to intense light, without eye protection, to prove that a creature infecting a host can be killed.

The Enterprise floods the colony with ultraviolet light from an array of satellites, killing the creatures on the planet and purging the parasites from the survivors.

As the Enterprise prepares to leave orbit, Spock reveals that his Vulcan inner eyelids had prevented permanent blindness and that he can see again.

Author James Blish used this draft of the script when adapting the episode for print, and this ending appears in the chapter "Operation -- Annihilate!"

The scene where the Enterprise landing party beams down to the surface was filmed at the TRW campus, Space Park (now Northrop Grumman Aerospace Systems) in Redondo Beach, California.

[5][6] The exterior of Sam Kirk's laboratory, which features white pylons, is Schoenberg Hall at the University of California, Los Angeles.

[9] This left actor Hundley with no lines in the finished episode; the character of Peter Kirk spends the entire show unconscious.

Club gave the episode a 'B+' rating, describing it as "a little rocky in places" but otherwise praising the alien menace and Nimoy's acting.

Historic quad courtyard plaza at Space Park with 75'×75' (23×23 m) reflecting pool (now filled with dry gravel for water conservation) where the crew beamed down from the starship Enterprise .
Bldg. S ("service") at Space Park showing staircase leading down to S-Cafe cafeteria. Denevans descended this to attack the Enterprise crew under the walkway seen in the center leading to the 1st-floor entrance. Also visible is the red-tiled wall near the lower-level entrance where Mr. Spock engaged in hand-to-hand combat with a Denevan.