The Enterprise traces a virus-like outbreak that seems to be traveling in a direct line across a planetary system. The next planet is home to Kirk's brother Sam, his sister-in-law and their young son. The Enterprise arrives too late however for Sam. They find flying jellyfish-like creatures that attach themselves to humans. They take over the victims nervous system forcing them to bend to their will. Spock finds a weapon to use against the creatures but it leaves him hopelessly blind. Written by
Did You Know?
This is one of only three original series episodes in which the word "damn" is used, with McCoy cursing the "damnable logic" behind the experiments designed to destroy the creatures. The only other uses of the word "damn" occur in Star Trek: Journey to Babel
(1967), wherein Kirk admits he can't "damn" Spock for his loyalty by taking command of the Enterprise at the cost of Sarek's life, and in Star Trek: Court Martial
(1967) where Kirk makes a log entry stating that the evidence against him is "damning." See more
In the beginning scenes on the bridge after Uhura raises Kirk's sister-in-law, Uhura is not sitting at the communications station which places Spock's science station to her left; Spock's station can be seen in the background to her right which would place her at the navigation sub-station which is to the left of the science station. When she loses Aurelen's signal and Kirk walks away, she is now sitting at her correct station to the right of the science station. See more
[holding a beaker of tentacle tissue
Evidently, when the creature attacks it, uh, leaves a stinger much like a bee or a wasp, leaving one of these in the victim's body, and it takes over the victim very rapidly, and the entwining is far, far too involved for conventional surgery to remove.