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?
The fly-by of the Enterprise that opens this episode was only seen one other time. It is re-used in Star Trek: The Tholian Web
(1968) as the ship is thrown clear of the Tholian force field. See more
When Spock stuns the creature to take as a specimen, it lands face down, but close-ups show it face up. See more
Freeze right there, Mr. Spock, or I'll put ya to sleep for sure.
Referenced in Free Enterprise