Season synopsis
Contains spoilers for this season and earlier seasons.
The Enterprise begins its five-year mission as a working starship crew facing problems that rarely have one simple answer. In “Balance of Terror,” a Romulan vessel destroys Federation outposts while hiding behind a cloaking device. Kirk pursues it as it retreats, and the two captains try to anticipate each other’s moves without knowing whether the other ship is close enough to strike. A weapons officer’s suspicion of Spock grows from the Romulans’ resemblance to Vulcans, but Spock’s actions save him and the ship. The Romulan commander, rather than surrender, destroys his own vessel after acknowledging the respect he has developed for Kirk.
Other missions turn the crew’s choices into more personal tests. When McCoy’s accidental overdose sends him through the Guardian of Forever and erases the Enterprise from history, Kirk and Spock follow him to Depression-era Earth. They find Edith Keeler, whose peace movement would delay America’s entry into the war and alter the outcome of history. Kirk falls in love with her, yet when McCoy is about to save her from a fatal accident, Kirk stops him so the timeline can be restored. The officers return through the Guardian knowing that their rescue depended on allowing her death.
A transporter accident splits Kirk into two versions of himself. One becomes indecisive, while the other attacks Yeoman Rand and hides aboard the ship. Scotty and Spock test a repair on an animal that was also split, but it returns whole and dead. With the landing party nearly frozen on the surface, Kirk gambles on recombining before McCoy can determine why the animal died. The procedure restores him, allowing the stranded crew to be rescued.