Όταν ο Σποκ κολλάει μια θανάσιμη αρρώστια για τους Βουλκάνιους, το Εντερπράιζ σπεύδει να συναντήσει ένα σκάφος που μεταφέρει το μόνο φάρμακο που μπορεί να τη θεραπεύσει.
The Enterprise has as an observer on their current exploration/contact missions: Commander Ari bn Bem of the planet Pandro. He is rude and obnoxious and dangerously independent, but Kirk is under orders to treat him well to further relations with Pandro. After six months in his cabin Bem insists on going on a landing party. Trying to rescue Bem (who claims he is observing the primitive locals), Kirk and Spock are captured. Bem has replaced their communicators and phasers with dummies to observe them under stress as well. He reveals...
The Enterprise explores a mysterious cloud in space. After passing through it the ship's computer begins acting in a bizarre fashion. It replicates Kirk's shirts with ""Kirk is a Jerk"" on the back, manufacturers devices that leave a black eye ring on Spock, and shoot cream pies out of the food devices. Things take a turn for the worse when McCoy, Sulu, and Uhura are trapped in thea holodeck where they are beset by dangerous weather phenomena. They are rescued by Scotty when he bypasses the controls, but the Enterprise...
Dr. McCoy is arrested and held responsible for causing a plague that ravaged the planet Dramia Two, where he performed a mass-innoculation 19 years earlier.
The Enterprise is tracing the course of a probe that scanned Earth and sent a signal back along its course. They are attacked by a serpent-shaped energy field belonging to a feathered serpent who claims to be Kulkukan of Mayan-Aztec legend. It beams Kirk, McCoy, Scott, and Ensign Walking Bear aboard its ship to solve its puzzle before he will reveal himself. Aligning the mirrors in a strange city, they solve the puzzle and Kulkukan appears before them. It takes them to its ""zoo"" of violent creatures that now live...
With Commodore Robert April (the first commander of the Enterprise) on board, Kirk pursues a ship plunging into a supernova, and enters a universe where time runs backwards.