Spock begins to undergo Pon Farr, the Vulcan mating cycle, forcing Kirk to set the Enterprise on a course for Vulcan.
Who Mourns for Adonais?
A powerful alien halts the Enterprise in its tracks and claims to be the ancient Greek god Apollo.
Kirk becomes the last line of defense against Nomad, a mysterious Earth probe that had been altered and given a directive to destroy anything that is imperfect.
Kirk, McCoy, Scotty, and Uhura are beamed into a parallel universe, where murder and deceit are the common ways of its barbaric Starfleet.
While scouting a peaceful-seeming world, the Enterprise landing party finds a primitive civilization that worships a computerized entity, named Vaal, as a god.
The Doomsday Machine
The Enterprise finds the crippled USS Constellation, which had been severely damaged in a battle with a powerful planet-killing doomsday weapon.
While looking for two missing officers, the landing party becomes the captives of aliens holding a great power of illusion.
The Enterprise is commandeered by an android, which pilots the ship to a planet of androids who are in cahoots with the mischievous Harry Mudd.
Kirk, Spock, McCoy, and a Federation negotiator become stranded when their shuttle is sucked into a celestial body by a mysterious force that has also somehow revitalized Zephram Cochrane, the man who invented warp drive two centuries before.
Journey to Babel
Spock's parents, Vulcan ambassador Sarek and his wife, board the Enterprise for a Federation conference. But trouble arises when one of the delegates is murdered--and Sarek becomes a suspect.
While trying to negotiate trade with a planet's warlike tribal society, the Enterprise landing party is taken prisoner by one of the factions, which is dealing with the Klingons.
The Deadly Years
A disease that causes rapid aging infects several members of the Enterprise crew, and eventually leaves the captain senile and unable to command the ship.
Kirk becomes convinced that a deadly gaseous cloud is an intelligent being--the same that killed 200 crew members aboard the USS Farragut, on which Kirk served 11 years earlier.
Wolf in the Fold
While taking some time for R&R at a port on a peaceful alien planet, Scotty becomes the suspect in a series of murdered women.
The Trouble With Tribbles
While investigating a disturbance at space station K-7, the Enterprise crew comes face to face with a group of Klingons and an even larger group of small, furry creatures known as tribbles.
The Gamesters of Triskelion
Kirk, Uhura, and Chekov are kidnapped to the planet Triskelion and forced to engage in arena games to the amusement of three powerful aliens who bet on the outcomes of the fights.
A Piece of the Action
Hoping to undo social damage caused by a visiting starship a century before, Kirk, Spock, and McCoy beam down to a planet whose society is based on the Chicago gangsters of the 1920s.
The Immunity Syndrome
The ship encounters a huge, single-celled lifeform in space, and Kirk is forced to send either Spock or McCoy on a suicide shuttle mission to investigate methods to destroy it.
A Private Little War
When the Klingons arm the aggressing side of a primitive culture with firearms, Kirk takes it upon himself to arm the other side in an effort to level the playing field.
Return to Tomorrow
Kirk loans the use of his body and those of two of his crew members to aliens who exist as pure energy, so they can build robot bodies to inhabit.
Patterns of Force
Investigating the disappearance of a missing Starfleet officer, an Enterprise landing party beams down to a planet modeled on Nazi Germany.
By Any Other Name
Aliens who have taken human form hijack the Enterprise to use as transport back to their home galaxy.
The Omega Glory
Searching for the missing crew of a Federation starship, a landing party beams down to find its captain, who has broken the Prime Directive in his obsessive search for a fountain of youth.
The Ultimate Computer
The Enterprise becomes the test subject for a new starship computer system, which if successful could render most of the crew obsolete.
Bread and Circuses
A landing party beams down to a parallel Earth that resembles a "20th century Rome," upon which Kirk, Spock, and McCoy are taken prisoner.
Researching history in the 20th century, the Enterprise becomes entangled with a mysterious man who is attempting to alter a historic crisis.