Ep 29. Operation Annihilate!

The Enterprise is on a mission to investigate the destruction of the Deneva colony. When they arrive, they discover that the colonists have all been rendered blind, and that the colony is under attack by a type of alien parasite that attaches itself to the nervous system of its host. The parasites appear to be controlling the colony's inhabitants, causing them to act in bizarre and aggressive ways.

After beaming down to the colony to investigate, Kirk, Spock, and McCoy are all infected by the parasites. McCoy is able to come up with a potential cure by using light and sound, and devises a plan to destroy all the parasites on the planet.

However, in order to do so, they must first subject themselves to massive amounts of pain. Kirk, Spock, and McCoy must all wear special belts that transmit powerful electrical shocks to their bodies. The pain is so intense that Kirk nearly loses consciousness, but they are ultimately successful in destroying the parasites and rescuing the remaining colonists.

The episode deals with themes of sacrifice and selflessness, as the crew of the Enterprise put themselves in harm's way in order to save others. There are also elements of tragedy and horror, as the parasites are shown to be extremely brutal and effective at manipulating their hosts.

Overall, "Operation Annihilate!" is a thrilling and thought-provoking episode that explores the boundaries of loyalty and bravery in the face of overwhelming danger.