9/16/1967. Amok Time — Spock begins to undergo Pon Farr, the Vulcan mating cycle, forcing Kirk to set the Enterprise on a course for Vulcan.
9/22/1967. Who Mourns for Adonais? — A powerful alien halts the Enterprise in its tracks and claims to be the ancient Greek god Apollo.
9/29/1967. The Changeling — 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.
10/6/1967. Mirror, Mirror — Kirk, McCoy, Scotty, and Uhura are beamed into a parallel universe, where murder and deceit are the common ways of its barbaric Starfleet.
10/13/1967. The Apple — While scouting a peaceful-seeming world, the Enterprise landing party finds a primitive civilization that worships a computerized entity, named Vaal, as a god.
10/20/1967. 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.
10/27/1967. Catspaw — While looking for two missing officers, the landing party becomes the captives of aliens holding a great power of illusion.
11/3/1967. I, Mudd — 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.
11/10/1967. Metamorphosis — 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.
11/17/1967. 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.
12/1/1967. Friday's Child — 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.
12/8/1967. 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.
12/15/1967. Obsession — 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.
12/22/1967. 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.
12/29/1967. 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.
1/5/1968. 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.
1/12/1968. 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.
1/19/1968. 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.
2/2/1968. 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.
2/9/1968. 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.
2/16/1968. Patterns of Force — Investigating the disappearance of a missing Starfleet officer, an Enterprise landing party beams down to a planet modeled on Nazi Germany.
2/23/1968. By Any Other Name — Aliens who have taken human form hijack the Enterprise to use as transport back to their home galaxy.
3/1/1968. 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.
3/8/1968. 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.
3/15/1968. 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.
3/29/1968. Assignment: Earth — Researching history in the 20th century, the Enterprise becomes entangled with a mysterious man who is attempting to alter a historic crisis.