By using the Guardian of Forever, Kirk, Spock, and the historian Erikson explore the planet Orion’s past. However, upon returning to the present, no one seems to recognize Mr. Spock; the U.S.S. Enterprise first officer for the past five years has been an Andorian named Thelin. A search of the U.S.S. Enterprise’s memory bank reveals that Spock, the son of Sarek and Amanda died while still a child; his death caused the couple to breakup.Spock recalls that during his “kahs-wan” — a Vulcan coming-of-age ritual — Selek, Spock’s distant cousin, had saved his life. Spock realizes that to put the present right again he must return to the past and become his own cousin to save himself.Returning to the Vulcan of his childhood through the Guardian, Spock presents himself to his younger self as Selek, and subsequently saves the boy’s life. In the process young Spock’s pet “sehlat” is mortally injured. The boy chooses to compassionately end his pet’s suffering and in doing so sets the young half-human, half-Vulcan on a path toward embracing Vulcan traditions. Upon returning to the present, Spock finds the timeline and his place on the U.S.S. Enterprise restored.
The Enterprise crew learns that a massive, planet-destroying extragalactic entity has entered Federation space and threatens every planet in the galaxy. The starship intercepts the gaseous form in an inhabited planetary sector. Almost immediately the cloud envelops the planet Alondra, breaking it into small pieces in seconds. The cloud proceeds on a direct course for planet Mantilles, inhabited by nearly 100 million people. Due to its proximity to the cloud, the Enterprise is pulled inside and is threatened by an antimatter barrage.The ship’s shields barely hold together while Spock discovers that the cloud is a living entity with a brain. He also discovers that they can escape through the top of the entity if they act quickly. The Enterprise travels through the creature’s body while antimatter continues to drain the ship’s power. Scotty, however, is able to create a power regeneration system by using part of the entity’s body. Kirk realizes that the only way to save the planet may be to kill the creature by exploding the Enterprise while inside. Instead, Spock mind-melds with the cloud and allows the creature to see glimpses of human life. The creature comes to understand that its food — planets — contains living creatures. The cloud allows the Enterprise to leave peacefully and agrees to depart the galaxy.
The Enterprise enters an uncharted sector of space where Federation and Klingon vessels have been reported missing every 27 years. While in the Taurean system signals begin to affect the male crew, causing them to hallucinate about beautiful women in peaceful surroundings. Kirk, Spock, and McCoy beam down to the source of the signals. After arriving on the planet they are taken prisoner and rendered unconscious by a group of beautiful, golden skinned women. Scotty takes command of Enterprise but begins to act peculiar himself.Upon waking, Kirk, Spock, and McCoy discover that they are aging rapidly as a result of life-draining headbands they are forced to wear. McCoy administers cortropine to counter the aging process while Spock, having obtained a communicator, unsuccessfully tries to contact the ship.On board the Enterprise, Uhura has taken command due to Scotty’s irrational behavior, while Nurse Chapel acts as Chief Medical Officer. Uhura orders an all-female landing party to transport down to rescue the men. The siren women explain to them that they had fled from a dying planet only to arrive on Taurus II where they encountered the life-draining force. The women developed a biological resistance to the force, effective only if they can replenish their own life-force every 27 years. Thus they capture men to drain their energy. The landing party eventually beams the male landing party back up to the ship just in time to replenish their life-force. Kirk agrees to transport the women to a safe world where they will again prosper.
While returning to a famine-ridden Sherman’s planet with a cargo of quadrotriticale, the U.S.S. Enterprise encounters a Klingon warship, commanded by Captain Koloth, who is detaining the trader Cyrano Jones. Kirk rescues Jones from the Klingon vessel and discovers him to be carrying a new breed of tribbles which have been genetically altered to slow their reproductive process. However, these tribbles are unable to resist the cargo of quadrotriticale the U.S.S. Enterprise is carrying. The U.S.S. Enterprise crew soon realize that the consumption causes the tribbles to grow to a tremendous size. So much so, that Jones’ tribble-eating glommer is unable to devour them.The Klingons return and demand that Kirk hand over Jones, accusing Jones of stealing the glommer from them. Instead of handing Jones over to the Klingons, Kirk returns the animal along with some of the tribbles.
Near the Romulan/Federation neutral zone, a long-lost philanthropist, Carter Winston, is discovered on board a damaged one-man vessel. He is reunited with his fiancee, Lieutenant Anne Nored, on board the Enterprise. Immediately following their reunion, Winston tells her that he has changed and can no longer marry her. The Enterprise computer reveals that Winston is actually a shape-shifting Vendorian. Winston eventually takes the Captain’s form and orders Sulu to steer the Enterprise into the Neutral Zone. Kirk discovers the ruse and orders the ship out of the Neutral Zone.Romulan ships, having been advised of the breach of the Neutral Zone, surround the Enterprise. The Vendorian, disguised as an Enterprise crewman, damages the deflector shield, which requires two hours to repair. Later , the creature reveals his identity to Nored, explaining that her former fiancee is dead but since he had absorbed so many emotions from human males, he indeed can love her.The Romulans proceed to attack the U.S.S. Enterprise. Realizing that he has put his love in jeopardy by destroying the ship’s shields, the shape-shifter forms himself into a deflector shield around the ship. After the Romulans retreat, the Vendorian reveals that he is a spy for the Romulan Empire, but he could not allow Nored to die in battle. Kirk informs him that he will stand trial but his efforts to save the ship will be considered. Nored tells him that he is much like her former lover and that he will enjoy life on Earth, perhaps even together with her.
While visiting the recently discovered planet Phylos, the U.S.S. Enterprise landing party discovers an apparently abandoned city. While exploring the city Sulu picks up a plant and is poisoned. As the landing party tries to save Sulu, the alien species that inhabit the planet approach and save Sulu’s life. Upon further investigation of the alien creatures, who are plantlike beings called Phylosians, the crew discovers that the poison that affected Sulu was accidentally brought to the planet by a visitor who subsequently worked to find a cure. The Phylosians call this visitor “Master” and “Savior”.The landing party eventually discover that this visitor was actually a Terran scientist by the name of Dr. Starros Keniclius who has since died. Now a giant clone of Dr. Starros Keniclius, by the name of Keniclius Five, has taken his place. The giant wishes to clone Spock to create an intergalactic peacemaker. Ignoring Kirk’s insistence that the Universe has become a virtually peaceful conglomeration of worlds, Keniclius Five clones Spock, crippling the Vulcan almost to the point of death. Kirk realizes the only way to save Spock is to recite aspects of Vulcan philosophy on life and death to Keniclius Five. Kirk convinces the clone that the new Spock is but a copy and not the original. The clone Spock saves the other’s life through a Vulcan mind meld and the duplicate remains on the planet with Keniclius to revitalize that world’s civilization.
While investigating the theory of creation, the Enterprise is caught inside an energy/matter tornado and pulled to the center of the galaxy. All computer systems fail and the crew begins to lose consciousness when a devil-like creature named Lucien appears on the bridge. The creature repairs the ship’s systems and takes the crew to explore his planet, Megas-Tu, but almost immediately returns them to the ship.The crew begin to experiment with magic after the encounter, much to Spock’s dismay. The Megans begin to take notice of the crew and return to the ship in phantasm form. They return the Enterprise bridge crew to their planet in a witch-hunt trial where Lucien is accused of bringing evil to Megas-Tu.Spock and Kirk defend the crew and Lucien, but their arguments fail and Kirk is forced to use his new magic against the prosecutor. Kirk explains that the Megans can only win if they become like the Earthlings they fear. Kirk then tells the jury that he will give his own life for an alien’s rights, which impresses the Megans. They learn that Lucien was the basis for Earth’s original conception of the devil, and the Megans, realizing that the Enterprise’s visit to their dimension was an accident, help the crew to return to their galaxy.
The U.S.S. Enterprise returns to the amusement planet where the crew had once taken shore leave. The crew begin imagining their fantasies when things go wrong: McCoy envisions a southern Earth plantation, but instead the Queen of Hearts appears screaming “Off with his head!” followed by the arrival of a spear throwing deck of cards. When Uhura tries to rescue McCoy, she is captured by a hovercraft robot. McCoy informs Kirk of the dangerous situation, whereupon all crew members are recalled to the starship.Realizing that Uhura is still trapped on the planet, Kirk attempts to contact the planet’s Keeper. Kirk’s landing party discovers the Keeper’s tombstone, and notices that the planet, lacking the Keeper’s influence, has become a dangerous world run by robots. The robots begin to pester the crew and the orbiting Enterprise.Deep underground, Uhura tries to outwit the planet’s master computer, while Kirk and Spock trick it into allowing them passage into the inner caves. The crew convince the computer of its usefulness as a peaceful and creative conscience and it ends the malfunctions on the planet and starship. While the crew returns to enjoy their shore leave, Spock continues discussions with the master computer.
The U.S.S. Enterprise receives orders to find Harry Mudd, a Federation outlaw, on the planet Motherlode; Mudd is accused of selling a fake love potion to miners. Upon the U.S.S. Enterprise’s arrival, Mudd demonstrates the potion on a woman who Spock discovers to actually be a reptilian illusion. Angered by this the miners begin to attack Mudd, but he is rescued by the U.S.S. Enterprise. Aboard ship, and after convincing Nurse Chapel to try the potion on Spock, Mudd steals a shuttlecraft. When Chapel discovers that the potion doesn’t actually work, she becomes bitter and sets out to confront Mudd. During a fight between Mudd and Chapel, special love crystals are broken near an air vent and permeate throughout the ship.Without warning, the delayed potion begins to affect Spock. Shortly, the potion begins to affect the entire crew. Spock becomes so enamored with Chapel that he will do anything to rescue her. Kirk accompanies him to find Mudd, but are attacked on the planet by large rock creatures. Unable to transport to the vessel because of the potion’s affect on the crew, Kirk uses the crystals to make the creatures become docile and loving. Eventually the potion wears off and the crew beam everyone back to the ship, but then the potion’s after effects cause hatred and animosity among the crew. Mudd is placed in the brig, where he confesses to other con jobs and is once again sent away for rehabilitation.
The U.S.S. Enterprise receives a garbled message while exploring the dead supernova, Arachna. The only comprehensible portion of the signal, which emanates from the star Cepheus, is the word “terratin.” Upon investigation the U.S.S. Enterprise encounters a planet of crystalline structure, barraged by volcanic activity. While in orbit, a flash of light passes through the ship, paralyzing the crew.Then the crew begins to realize that all organic matter on the ship is shrinking; all body molecules are degenerating. Spock calculates that in a matter of minutes the entire crew will stand centimeters tall. By using man-made ladders, the crew gets to the bridge consoles and use their collective strength to press the transporter buttons. Kirk transports to the planet’s surface and immediately grows to full size, since the transporter can change all matter to previously detailed size and shape. While on the planet, Kirk encounters a miniature city, but is shortly beamed back to the ship by the transporter’s automatic settings.Upon returning, he discovers that his crew has been transported off the ship to the planet surface. Kirk threatens the miniature city, demanding the crew’s return. The leader informs Kirk that the lightning attack was the only way to get the attention of the starship, because their city is being threatened by volcanic activity. Spock discovers the Terratin natives to be mutated descendants of a missing scientific expedition; the survivors had named the planet Terra 10. The crew is transported back to the ship, but since the Terratins cannot be restored to their original size, they and their entire city are beamed aboard the U.S.S. Enterprise to be taken to a more stable planet.
While exploring the Delta Triangle, where many starships have disappeared, the Enterprise is attacked by several Klingon vessels. During the fight, the Enterprise and a Klingon ship slip into a space-time warp where a starship graveyard is discovered to hold ships from all over the galaxy. The Klingon ship fires at the Enterprise but the torpedo is neutralized and the two ships captains are brought to a trial by the inhabitants of this dimension, called Elysia. The council is made up of all races from across the universe and lead by a Vulcan, Xerius. He explains that no one has been able to escape from this area, so the descendants of the crews have learned to co-exist in a peaceful state. Violence is not permitted and all weaponry is immediately neutralized.Scotty discovers that the timewarp is disintegrating the Enterprise’s dilithium crystals, and insists that they must leave the trap soon or they will be powerless. The Klingons agree to link ships and cooperate by sharing warp power and personnel to escape. However, Spock learns that the Klingon Captain, Kor, wants to destroy the Enterprise when they break free of the time warp by using a specially calibrated explosive device in the warp drive. The Enterprise and Klingon crew celebrate their impending escape but when a Klingon slips away to plant the explosive, Kirk stops her and diffuses the bomb. The two ships leave the timewarp and separate, with Kor claiming full credit for saving the two vessels.
The U.S.S. Enterprise arrives to study the planet Argo — a water-based planet where earthquakes caused the land masses to sink — for the benefit of other Federation worlds that will encounter similar quakes. The crew lands in a specially designed aqua-shuttle but is immediately attacked by a sea monster and Kirk and Spock become trapped in the monster’s grip.Five days later, McCoy, Scott, and Lt. Clayton find the missing officers; they have been transformed into water breathers by the monster. Kirk and Spock explore the submerged Argo and find a group of inhabitants who hate any air-breathers. Hesitantly, the Aquans agree to perform surgery to return Kirk and Spock to normal, but when Scotty dives down to warn of an impending quake, they become fearful of air-breathing spies. Kirk and Spock are deemed treacherous and are sentenced to death. Rila, a water breather, agrees to help them if they will in turn help him find a buried city and a way to reverse the mutation.They are successful and obtain the necessary medicine to help Kirk and Spock, but Kirk nearly dies from the vaccination. In orbit, the U.S.S. Enterprise uses its phasers to hit the uninhabited sections of the planet in an attempt to move the quake’s epicenter away from the Aquan city. Grateful for saving their lives, the young Aquans make their records available to the Federation and decide to use the new found medicine to reverse their water-breathing mutations and resume life above ground.
In the shuttlecraft Copernicus, Spock, Uhura, and Sulu are en-route to Starbase 25 to deliver a stasis box — a rare artifact of the Slaver culture — which stops time inside so any object can last forever. The now-extinct Slavers used these objects to carry weapons, valuables, and other scientific instruments and data. The boxes can detect each other and evidence shows that another device is located nearby. Spock begins to investigate. The shuttle lands on an ice planet where hostile Kzinti lie in ambush with an empty stasis box. The Kzinti once controlled the entire empire but now are bound to keeping peace. However, many Kzinti still take part in raiding Federation shipping and seek Slaver weapons to regain control of their empire.The Kzinti take the shuttle crew hostage in an effort to take the box and its contents. The Kzinti leader obtains the weapon, but while tampering with it he enables the crew to escape. Spock steals back the weapon, but Uhura is recaptured. After Sulu creates an explosion with the weapon, it is recaptured by the Kzinti, who are killed when the device self-destructs.
Responding to radio emissions near the edge of the known universe, the U.S.S. Enterprise is dragged off course by Questar M-17, an imploded negative star mass. The Enterprise manages to establish itself in orbit around Questar M-17 and encounters a strange starship inhabited by an evil shapeless alien. Further investigation reveals that the ship’s engineers had failed to remove the creature before attempting to destroy their own vessel. Eventually, the creature takes control of the Enterprise. Kirk and Spock devise a plan to steer the U.S.S. Enterprise toward Questar M-17, tricking the creature into believing that the Enterprise will be destroyed. The creature flees for its life while the Enterprise frees itself from Questar’s gravity pull by employing the slingshot effect, leaving the creature to orbit forever around the star.
The disappearance of a scientific team lures the U.S.S. Enterprise to investigate near Lactra VII. The Starship Ariel is located there, abandoned, with the Captain having transported to the planet’s surface. The U.S.S. Enterprise crew beams down to discover a series of unusually juxtaposed environments. On the planet the crew meets the Lactrans, a group of twenty foot slugs with intelligent capacities far beyond their own; they capture the U.S.S. Enterprise crew and imprison them along with the three remaining Ariel crew members. Ariel’s Lieutenant Randolph has become ill, but the Lactrans are unaware, as their methods of communication are highly sophisticated.Spock can partially read the Lactran thoughts; he realizes that they are much more advanced than Vulcans and they are amused by his attempts at communication. Spock tricks a Lactran child to bring him a communicator and medical kit that are located outside a force field. Kirk contacts the ship and they are beamed up along with the child. On the starship, the child reads all the information contained in the library computer, but the child’s parents are worried and almost kill Kirk in an attempt to contact him. Scotty beams down with the child, who learns that humans are not pets for Lactran amusement. The child informs the other Lactrans, who release the crew from confinement and the ships leaves with a message from the Lactrans — “humans may return when they grow up, in 20 or 30 centuries.”
The U.S.S. Enterprise arrives at the Vedala asteroid, where Kirk and Spock are summoned to learn about a stolen religious artifact that could ignite a galaxy-wide holy war, a jihad. A team of specialists are called in to recover the item with Kirk and Spock: Tchar — the hereditary prince of the Skorr, Sord — a powerful reptilian, a master lockpick — Em/3/Green, and Lara — a great hunter.The Vedalan tells the team that the artifact dates back two centuries, when Tchar’s people were fighters and a religious leader, Alar, came to teach fundamentals of peace. Alar’s brain patterns were preserved in the sculpture, the “Soul of the Skorr.” Now the device has been stolen and is hidden on the very unstable “mad planet.” The Vedala transports the group to the planet where they encounter harsh weather, quakes, volcanic lava flow, and severe winds. After barely escaping a lava river, the team finds itself nearing a position that Tchar had originally spotted from the air.Tchar flies ahead to investigate the fortress in which the object is kept. Meanwhile, Kirk and Lara scout ahead to explore the terrain. Rejoining the group, they head for the fortress on foot to rendezvous with Tchar. Upon reaching the structure, Em/3/Green picks the lock, but the team is attacked by a group of mechanical birds who are protecting the building. They narrowly make it inside and after surviving a set of traps, they find the sculpture, but conclude that it must have been Tchar who originally stole the object. Tchar admits his treachery, saying that his people have become a docile race, and the ensuing war will make them great again. Tchar tells them that their mission must always remain secret so that no one will learn about the treachery. Kirk and Spock return to the U.S.S. Enterprise noting that, remarkably, only two minutes had passed since they beamed to the asteroid.