Episode: 030 | Aired: September 14, 1967
Spock experiences "pon farr," the highly ritualized Vulcan mating cycle, and must return to Vulcan to take a wife, or die. His betrothed since childhood, T'Pring, rejects Spock in favor of Stonn, a full Vulcan. She demands that Spock fight a champion of her choosing to win her, and shocks all with her selection of combatant.
Episode: 031 | Aired: September 21, 1967
The Enterprise is held captive by a powerful alien who was once worshipped as the Greek God Apollo on Earth. The being wants to force the crew to submit to him, and their attempts to foil his plans are hampered by a young Lieutenant who has fallen in love with him and will be his bride.
Episode: 032 | Aired: September 28, 1967
The Enterprise encounters an ancient space probe launched from 21st century Earth called Nomad. The probe is responsible for the destruction of an entire star system, but spares the Enterprise, believing that Kirk is its creator. When the probe begins to attack members of the crew, Kirk must find a way to disable it.
Episode: 033 | Aired: October 05, 1967
A transporter malfunction switches Kirk, Scott, Uhura and McCoy with their counterparts in a savage alternate dimension, where power is attained through assassination and the Enterprise is poised to destroy a world.
Episode: 034 | Aired: October 12, 1967
Planet Gamma Trianguli VI is a seemingly idyllic paradise inhabited by immortal, ageless, child-like people. The crew quickly discovers, however, that the planet is deadly and ruled by a god that demands daily offerings.
Episode: 035 | Aired: October 19, 1967
Having faced off with an ancient alien planet-eating robot ship and lost, Commodore Matthew Decker pulls rank on Kirk and puts the Enterprise at risk to destroy the machine.
Episode: 036 | Aired: October 26, 1967
On the planet Pyris VII, the Enterprise crew finds a gothic world of witches, black cats and haunted castles, and is victimized by a pair of aliens bent on conquest.
Episode: 037 | Aired: November 02, 1967
On an unmapped planet, the crew finds con man Harry Mudd surrounded by hundreds of beautiful female androids who exist to serve humans and cater to his every whim.
Episode: 038 | Aired: November 09, 1967
While transporting Assistant Federation Commissioner Nancy Hedford to the Enterprise so she can be treated for a terminal condition, Kirk and his crew discover Zefram Cochrane, the legendary Federation. Cochrane has been kept alive by an alien energy form he calls the Companion, but when the Enterprise crew attempts to leave, the entity reveals itself to be less than completely altruistic.
Episode: 039 | Aired: November 16, 1967
The Enterprise is assigned to transport ambassadors to a peace meeting called the Babel Conference. Among the diplomats are Ambassador Sarek of Vulcan and his human wife, Amanda Spock's parents. Tensions and accusations cause Sarek to suffer the equivalent of a Vulcan heart attack, and Spock must help his estranged father or he will die.
Episode: 040 | Aired: November 30, 1967
Kirk, Spock and McCoy are sent to the planet Capella IV to negotiate a mining treaty, and find themselves facing off with the Klingons in efforts to protect the pregnant widow of the planet's murdered leader from an attempted coup.
Episode: 041 | Aired: December 07, 1967
Stopping to deliver supplies to the colonists of Gamma Hydra IV, the crew of the Enterprise becomes infected with an illness that causes rapid aging, incapacitating the command staff and leaving the ship vulnerable to Romulan attack.
Episode: 042 | Aired: December 14, 1967
Encountering for the second time a vampire-like cloud creature that drains the red blood cells from humans, Kirk defies orders and is determined to kill the creature before it can claim more lives.
Episode: 043 | Aired: December 21, 1967
Recovering from a head wound accidentally caused by a female crew member, Scotty unwinds in a nightclub on the planet Argelius II and leaves with a beautiful dancer. When the dancer is murdered, Scotty is found holding a bloody knife.
Episode: 044 | Aired: December 28, 1967
Annoyed at being sent to a space station to protect a shipment of quadrotriticale grain, Kirk uncovers a Klingon plot with the help of highly reproductive, furry little creatures called Tribbles.
Episode: 045 | Aired: January 04, 1968
Captain Kirk, Lieutenant Uhura and Ensign Chekov are abducted and forced to fight as gladiators on the planet Triskelion.
Episode: 046 | Aired: January 11, 1968
The inhabitants of planet Sigma Iotia II have modeled their culture after Earths Chicago mobsters of the 1920s, and in order to end the planets constant violence Kirk assumes the role of a powerful mob boss.
Episode: 047 | Aired: January 18, 1968
Rushing to investigate the fate of solar system Gamma 7A, which has gone radio silent, the Enterprise is confronted with an enormous single-celled space amoeba. To destroy the creature, which feeds on energy, Kirk must choose between Spock and McCoy for a suicide mission.
Episode: 048 | Aired: February 01, 1968
A peaceful society on a primitive planet is corrupted when Klingons provide the inhabitants with firearms.
Episode: 049 | Aired: February 08, 1968
Deep in a cave on an ancient planet, three highly intelligent survivors of a cataclysmic war exist as non-corporeal entities. Sargon, his wife, Thalassa, and their former enemy Henoch have been waiting for someone to find them, and now request use of three human bodies long enough to construct androids to inhabit.
Episode: 050 | Aired: February 15, 1968
The Enterprise travels to planet Ekos to investigate the disappearance of historian John Gill, an old Academy professor of Kirk's. When it appears that Gill has violated the Prime Directive of non-interference by introducing Nazi ideology to the planet, Kirk is determined to discover the truth.
Episode: 051 | Aired: February 22, 1968
A group of agents from the Kelvan Empire hijack the Enterprise to make the 300-year journey back to their home in the Andromeda galaxy. In order to take control of the starship, the Kelvans -- huge creatures with tentacles -- assume human form and transform all but essential crew into small mineral blocks.
Episode: 052 | Aired: February 29, 1968
On the planet Omega IV, the Enterprise discovers that Captain Roland Tracey, commander of the Exeter, has violated the Federation's Prime Directive of non-interference by using his phaser to protect the Kohms, Asian-like villagers, against barbarian raiders called the Yangs.
Episode: 053 | Aired: March 07, 1968
The Enterprise is chosen to test a new, highly advanced computer that is able to run a starship without human intervention. Kirks worries about being replaced by the artificially intelligent machine come to nothing when the computer attacks and destroys other Federation ships during a war game exercise.
Episode: 054 | Aired: March 14, 1968
Kirk, Spock and McCoy are captured by a society that has developed into a modern version of ancient Rome and are forced to fight gladiatorial games for the entertainment of the citizens.
Episode: 055 | Aired: March 28, 1968
Sent on a mission back in time, the Enterprise inadvertently beams aboard a man named Gary Seven, who claims to have been raised and trained by aliens to prevent Earth from destroying itself. When Seven escapes, Kirk and Spock follow him to circa 1968 New York City.