A dreaded disease, choriocytosis, strikes the U.S.S. Enterprise. All except Spock are cured, as the illness is fatal only to Vulcans. With the starship on course to a Starfleet Academy dedication ceremony on Deneb V, Spock realizes that he has as little as three days to live. The cure for the disease is strobolin, which is found on only a few worlds in the galaxy, the closest of which, Beta Canopus, is four days away.Review
The U.S.S. Potemkin obtains the drug and transports it to the U.S.S. Huron, en route to the Enterprise. Without warning, the U.S.S. Huron is attacked by a vessel of Orion designation and the drug and a cargo of dilithium crystals are stolen. The U.S.S. Enterprise encounters the Orion ship in a volatile asteroid field by detecting the dilithium crystals in the ship's cargo bay. Kirk attempts to negotiate for the drug in exchange for not mentioning the stolen dilithium crystals, but the Orions' suspect a ruse. Kirk agrees to meet the Orion captain on an asteroid knowing that it might be a trap, but worth the risk for Spock's life.
The Orion captain arrives carrying a bomb capable of destroying the entire asteroid but Scotty beams the explosive to the Enterprise and discharges it. Scotty then beams up the two captains and forces the Orion captain to stand trial for piracy. With the medicine in hand, Spock is saved.
The U.S.S. Enterprise is hosting Commander Ari bn Bem of planet Pandro as an independent observer of the Federation. Starfleet is eager to open diplomatic relations with the Pandronians, due to their advanced medical knowledge. Though he has not joined the crew on previous missions, Bem accompanies a landing party to explore Delta Theta III and, due to his non-violent nature, secretly replaces their weapons with non-functioning ones.Review
The planet is populated by preindustrial aborigines - the Federation is interested in gathering information about the world for research purposes. While walking through the dense terrain, the crew scans lifeforms emanating from the forest and Bem immediately runs toward them against direct orders. Bem passes through the thick rain forest by splitting into several body parts and floating through the cracks. Kirk and Spock split off but are unable to track Bem through the dense woods. They later discover that he has been captured by a group of angry aborigines.
Scotty and Sulu beam back to the U.S.S. Enterprise and discover a force field developing around the small village, but they cannot contact Kirk or Spock. At night, Kirk and Spock attempt to rescue Bem from the natives but are also captured. Bem explains that he took Starfleet equipment because he dislikes violence and wanted to see how the captain could react without such devices. He returns the devices to Kirk and Spock, who manage escape only to be paralyzed by a supernatural force surrounding the area.
The entity tells them not to interfere with her "children," the people of this world. Bem escapes and Kirk and Spock are again captured. The entity demands that they leave immediately but Kirk tells Spock that they cannot leave without Bem. They arrange a search party to beam down, now that the entity has permitted communications. Bem is found and he realizes that he has failed in his mission and must die. The entity tells him he will not be killed because people must learn from their errors. The crew returns to the ship, where Spock ponders the existence of such a god-like entity and how they are all really children to the universe.
While studying an asteroid, the U.S.S. Enterprise is attacked by three Romulan vessels which claim that the Federation ship has trespassed into Romulan space. The U.S.S. Enterprise hides from the Romulans in a gaseous energy field to avoid the close range barrage from the war ships.Review
The gas from the cloud enters the Enterprise's computer systems and affects the functions of the computer. When the ship leaves for repairs the computer proceeds to play practical jokes on the crew by altering various ship functions, so that drinks leak and silverware bends. Spock is humiliated by his own microscope, and Scott is pelted with fruits and pies from a food dispenser. The crew discovers the source of the jokes when outrageous laughter is heard over the intercom. Sulu, McCoy and Uhura are trapped in a recreation room and Scott is unable to fix the logic circuits because gravity has been shut off. Life support systems are replaced by laughing gas, which causes pain to Vulcans.
The ship returns itself to the Neutral Zone, thereby inciting a Romulan attack. The ship then plays a practical joke on the Romulans by creating a balloon-shaped like the U.S.S. Enterprise. Kirk tricks the ship back into the energy field, which is followed closely by the Romulans. While in the energy field the computer is returned to normal but the Romulan vessel begins to be affected by the same symptoms. Kirk decides to wait a bit before telling the Romulans how to remove their practical joker.
The U.S.S. Enterprise arrives at Dramia I to deliver medical supplies when Dr. McCoy is arrested for mass slaughter and imprisoned. Nineteen years prior, the doctor had created an inoculation program for a Saurian virus on Dramia II, but immediately following his departure a plague virtually wiped out the entire population.Review
Kirk, aware of the swift Dramian justice system, worries about McCoy's future while McCoy thinks that the plague may have been his fault. Kirk and Spock begin an investigation into the plague and realize they must go to Dramia II to draw some conclusions. En route, they are pursued by Demos, the Dramian head of security. The U.S.S. Enterprise entraps the Dramian ship and holds it impounded. Forcing Demos to accompany them, the ship passes through an orbiting aurora and the three beam down to the planet.
On the planet, they pursue a plague survivor, Kol-Tai. When they finally catch up to him, he tells them of McCoy's courage and ability to save the Dramians from the virus, 19 years ago. Kol-Tai agrees to testify of McCoy's behalf, but the crew must return to Dramia I quickly or it will be too late.
Complicating matters, Kol-Tai brings the plague on board the U.S.S. Enterprise, and infects the entire crew, except an immune Spock. Kol-Tai may not live long enough to testify, while Demos still believes McCoy guilty but has himself contracted the fatal disease. Spock illegally breaks McCoy out of prison to cure the plague on the U.S.S. Enterprise. On board McCoy discovers that he is innocent and that the aurora recently encountered is the actual cause of the plague. By using blood samples from the infected, McCoy cures the plague and shares the information with the Dramians. Demos and the Dramian government agrees to dismiss the unorthodox jailbreak and drop the charges.
A mysterious probe that scanned a Federation system and sent a signal into outer space is pursued by the U.S.S. Enterprise, which follows its propulsion trail to see if it can catch the probe. While tacking the probe, the Enterprise encounters an alien vessel that is surrounded by a magnificent energy field which proceeds to encompass the Federation ship. The alien ship transforms itself into the shape of a large feathered serpent which reveals itself as Kulkukan of ancient Mayan-Aztec legend. Upset over being forgotten by Earthlings, Kulkukan transports Kirk, Scott, McCoy, and Ensign Dawson Walking Bear to his ship to solve an enigmatic puzzle. If they succeed, Kulkukan will reveal himself. If they fail, the crew will all perish.Review
The four crew members are transported to an Earth-like city filled with many cultures and must force a light source to strike a certain space on an ancient pyramid. When they accomplish the task, Kulkukan appears as the winged serpent, alleging to be their master. Transported to an ominous "life-room," the crew members notice a variety of creatures from around the galaxy, including the ferocious, but now peaceful, Capellan power-cat. Kirk explains that they cannot accept Kulkukan as their master, despite his contributions to mankind.
On the U.S.S. Enterprise, Spock discovers a way to break the energy field surrounding the ship, distracting Kulkukan from the other crewmen. The men release the animals from their cages, thereby creating a chaos which Kulkukan is unable to control. The power-cat traps Kulkukan and prepares to kill him so Kulkukan is forced to reveal himself as a mortal. Kirk saves Kulkukan and thus teaches him a valuable lesson about human survival. Kulkukan leaves as an old, broken mortal, and Kirk decides that the price of infinite knowledge is too high to pay.
The U.S.S. Enterprise is escorting Commodore Robert April and his wife Dr. Sarah April, the first captain and chief medical officer of the U.S.S. Enterprise, to Commodore April's retirement ceremonies on Babel. While en route, an alien vessel passes the U.S.S. Enterprise at warp 36 toward heading directly into the Beta Niobe nova. Locking onto the alien ship with the tractor beam, the U.S.S. Enterprise is dragged along. Unable to break free, both ships fall into the nova and arrive in an antimatter universe where time flows backwards.Review
The alien ship's pilot, Karla Five, is an explorer from the reverse universe who accidentally became lost in our universe. He leads the U.S.S. Enterprise to his home world of Arret to help find a way back into normal space. On Arret, the crew encounter an old man, who is Karla Five's son, and a baby, his father. The crew begin to notice that they too are getting younger as they remain in the negative universe.
In time they discover a dead star that corresponds to the nova in the positive universe - it is the gateway for the U.S.S. Enterprise's return. Though Karla Five's ship is strong enough to pull the starship through the nova, the Enterprise crew is now too young to pilot it. Commodore April, now a young handsome man, takes command of the ship steering it back through the nova. Using the transporter, April restores the crew to their proper age. The Aprils consider remaining young, but instead decide that they have lived their lives to the fullest. Once back to their original ages, the Federation decides to change the mandatory retirement age and the Aprils are able to continue their careers.