The Enterprise discovers an ancient derelict alien ship orbiting a dead star. Upon investigation they discover that the crew destroyed themselves, rather than letting a malignant alien escape the dead star.
Guest Stars: James Doohan
as Transporter Chief Kyle, Pod Ship Commander, Engineer, Malignant Alien
An accident during a time travel excursion causes Spock to be replaced by an Andorian first officer. To restore the timeline, Spock must travel back to his childhood and make sure he survives a boyhood testing ritual.
An enormous space-going entity consumes whole planetary systems... and a Federation colony is next in its path. The Enterprise must brave its antimatter interior and find a way to stop it.
While exploring a mysterious section of space, the male crew are attracted by a mysterious signal. Kirk beams down with a landing party and encounters a race of seductive women, who harbor a dark secret.
The tribbles are back, and are causing problems for the Klingons.
The Enterprise rescues Carter Winston, a financier and philanthropist who has been missing for years. However, it soon becomes clear that Winston is harboring a secret of his own, and is allied with the Romulans.
The Enterprise discovers a planet inhabited by a once vast plant-based civilization that have been reduced to a few survivors. They are now led by Dr. Keniclius, a human grown to giant proportions... and he wants Spock to join him.
The Enterprise travels to the center of the galaxy only to discover themselves sucked into a dimension where the laws of time and space no longer apply, and "magic" functions for those capable of grasping its fundamentals.
The Enterprise travels to the shore leave planet for some well-deserved rest and relaxation. The planet can create organic androids based on the crew's imagination, but they soon discover that the planet has a hostile mind of its own.
Harcourt Fenton Mudd returns with a love potion which he claims can cause any male and female to fall in love. He inadvertently releases it into the Enterprise's ventilation system and they discover that his claims are true.
The Enterprise responds to an old-style distress signal from an unknown "Terratin" colony, and follow it to its source: an uninhabited planet about to shatter from internal stress. However, a mysterious beam strikes the ship and the crew begins to shrink... and shrink...
The Enterprise is sent on a mission to explore a "Bermuda Triangle of Space," only to find itself and a Klingon ship trapped in a timeless piece of eternity inhabited by other lost ships. The two ships must work together before their only exit back to real space closes for good.
While exploring an aquatic planet, Kirk and Spock are lost at sea. They are later found... but have been transformed into amphibious life forms. Now they must track down the planet's mysterious inhabitants and find a cure, or be left to live as freaks in the Federation.
Spock, Sulu, and Uhura are delivering a Slaver stasis box by shuttlecraft when they detect another box. Following the signal, they soon discover that a Kzinti privateer has set a trap. Capturing the Enterprise crew, they discover their box holds a rare find, a shape-shfiting Slaver weapon. Now Spock and the others must keep the Kzinti from returning to their government with the weapon.
The Enterprise go looking for a lost expedition and a landing party finds the missing crew: trapped in an alien zoo in which they themselves are soon taken as prisoners. Now they must somehow establish communications with their evolved slug-like captors who think of them as primitive animals.
Kirk and Spock are chosen by a mysterious race to lead an expedition of alien specialists to an "insane planet" and recover a missing artifact, the Soul of Alar, before the avian Skorr go on a galactic jihad.
Spock is stricken ill with a disease that is fatal to Vulcans. With time running out, the Enterprise must rendezvous with another ship carrying the cure. However, Orion pirates hijack the shipment and Kirk must deal with them before Spock dies.
The Enterprise plays host to Ambassador Bem, an arrogant member of a neutral race. However, Bem endangers all of their lives when he conducts a test of suitability upon Kirk and Spock during an exploratory mission on a primitive planet.
After avoiding a Romulan ambush by entering a strange energy cloud, the Enterprise finds itself at the mercy of an insane ship's computer that indulges in a series of increasingly aggressive practical jokes.
The Enterprise goes to the planet Dramia II to deliver medical supplies, only to have McCoy arrested and put on trial for deliberately spreading a disease that wiped out an entire colony. Now Kirk must clear his friend's name before the doctor is executed.
Backtracking a mysterious probe, the Enterprise confronts a ship in the form of a giant feathered serpent. Its captain, Kukulkan, insists on putting Kirk and others through a test to determine if they have grown sufficiently to receive his wisdom.
The Enterprise inadvertently locks a tractor beam on a ship diving into a nova, and finds itself pulled into a bizarre parallel universe where time flies backward and the crew becomes younger at an alarming rate.