The Enterprise encounters a planet whose culture is patterned on ancient Rome... and holds gladiatorial games that Kirk, Spock, and McCoy must fight in.
Starfleet uses the Enterprise to test a new super-sophisticated computer, but it soon develops a mind of its own.
The Enterprise investigates the disappearance of another starship and discovers a planet where the inhabitants are immortal... and engaged in a strange parallel of Earth's Cold War period.
The Enterprise's command crew must thwart an invasion by aliens from another galaxy who plan to conquer this one.
The Enterprise seeks out a historical researcher with whom the Federation has lost contact, and discovers he has contaminated a culture, remaking it into a near-duplicate of Nazi Germany.
The Enterprise discovers three discorporate intelligences who seek their help in gaining physical bodies... but one of them has plans of his own.
The Enterprise must destroy an enormous space amoeba before it reproduces and threatens known space.
The Enterprise travels to a planet suffering from cultural contamination from an earlier expedition--the inhabitants mimic the culture of 1920's gangland Chicago.
The Enterprise must deal with a creature from Kirk's past that feeds on human blood.
Kirk, Spock, and McCoy land on a primitive planet to negotiate a mining treaty, but soon find themselves involved with intrigue and must flee with a pregnant woman into the surrounding mountains.
Spock meets his estranged father when the Enterprise escorts a group of ambassadors to a conference on the planet Babel.
The Enterprise is taken over by a group of androids who are working for Kirk's old nemesis, Harry Mudd.
When a landing party disappears and one man is beamed up dead, Kirk, Spock, and McCoy investigate and meet a pair of aliens who seem capable of performing magic.
The Enterprise must stop an ancient doomsday weapon that is capable of destroying entire worlds... and has already totaled one Constellation-class cruiser.
Kirk attempts to assist a primitive people that are ruled over by a computer named Vaal.
The Enterprise encounters an ancient Earth probe bent on the sterilization of all life.
The Enterprise encounters an alien who claims to be the Greek god Apollo.