delivers a team of historians to the planet of the Guardian of Forever. As two historians monitor the Guardian, Kirk and Spock accompany Historian Erikson to an observation mission to Orion a thousand years in the past. However, when Spock comes through the portal, McCoy doesn't recognize him. Assuming it's a practical joke, the party beams up to the ship only to discover that Scotty doesn't recognize Spock either. However, Kirk has a surprise of his own when he doesn't recognize the First Officer of the Enterprise
, Thelin, an Andorian. Only Kirk and Erikson recognize Spock. Erikson goes over a log of their activities in the past but can't find any change that would have caused Spock's disappearance from the timeline. There is no record of a Spock serving in Starfleet, but they locate Spock's father Sarek. He is still an ambassador, but he and his wife Amanda were separated after the death of their seven-year-old son Spock, and Amanda died in a shuttlecraft accident while returning to Earth.
Kirk and Spock check with the two historians who monitored the Guardian, and they admit they were scanning Spock's past during the time of his "death." The records show that Spock died during a desert maturity ritual, the kahs-wan
. Spock remembers undergoing the ritual and surviving only because a distant cousin, Selek, rescued him when a wild animal attacked. Spock never saw Selek again and remembers that he looked like an adult Spock. They realize that in the original timeline, Spock went back and saved his younger self. However, since he was in Orion's past when the historians monitored Vulcan history through the Guardian of Forever, Spock couldn't go back to save himself, and thus his younger self died. The Guardian confirms their theory as well as the fact they can restore the timeline if Spock goes back in time. Although Spock notes his memories are not clear, he has no choice but to go back equipped with requisitioned clothing suitable to period. First Officer Thelin wishes him well, realizing that his timeline will be eliminated but understanding the importance of family.
Going from his memories, Spock enters the Guardian and arrives at the proper time in the past outside of his home city of ShirKahr. He arrives in the city and goes to the family home. Outside he witnesses Young Spock fighting with several young Vulcans taunting him for his half-human blood. They easily defeat him when he's unable to use the Vulcan nerve pinch on them. Sarek notices Spock and apologizes for his son. Spock notes it's a family matter he won't speak further of, and introduces himself as a distant cousin, Selek, on a trek to a religious shrine. Sarek offers him a room in the household and then wonders why Spock looks familiar to him. Inside, Sarek goes to see his son, who is with his pet sehlat, a large bear-like creature named I-Chaya. Sarek reminds Young Spock that he has been raised a Vulcan and will soon have to choose either the Vulcan or human way. Meanwhile, Amanda talks to Spock and apologizes for her son's behavior. Spock understands that Sarek expects perfection. Amanda is surprised that Spock understands Young Spock better than her husband does. Spock asks about the kahs-wan
and Amanda says that it will occur in a month. Spock is confused, unable to understand why the date is different then the one he recalls. Meanwhile, Sarek talks to Young Spock about the ritual and reminds him that failure the first time isn't normally a disgrace... for a normal Vulcan. Young Spock worries about what happens if he fails but Sarek tells him that won't happen.
Spock tries to recall the reason for the discrepancy while Young Spock sneaks away. The aging I-Chaya follows Young Spock despite his orders not to. Spock finally remembers that as a child he went on his own personal kahs-wan
to determine if he was a mature Vulcan. Spock goes into the desert after his young self, while Amanda and Sarek discover them both missing. Sarek is still puzzled by Spock's familiarity and unsure if he would abduct Young Spock, and calls the authorities for a search. In the desert, a le-matya attacks Young Spock but I-Chaya attacks it. The two animals struggle and I-Chaya goes down. Spock arrives and renders it unconscious with a neck pinch. Young Spock thanks him as they move away and wonders if he'll ever learn the Vulcan neck pinch. Spock assures him they will and asks why Young Spock went out into the desert. The boy explains that he didn't want to disappoint his father, and admits that his mother's emotions upset him. Spock knowledgeably notes that Young Spock sees his own emotional traits in his mother, and explains that Vulcans suppress their emotions with logic but still experience them. I-Chaya collapses and they realize it was scratched by the le-matya's poisonous claws. Spock realizes that in his timeline, I-Chaya didn't die. Only a healer would have the antidote, and Young Spock reaches the logical conclusion that he has to go into town to summon one. He departs while Spock stands vigil over his dying pet.
Young Spock avoid a carnivorous planet and goes to a healer. The healer suspects him of playing a prank but Young Spock quickly challenges him, saying no one has ever called him a liar. The healer takes him and goes out into the desert in a hovercar. While the healer examines I-Chaya, Spock praises the boy for his bravery, but Young Spock admits his concern was for I-Chaya. The healer informs them that it's too late to save I-Chaya, but can prolong his life at the cost of considerable pain. Young Spock makes the choice to honor his pet and grant him a quick painless death. They return to the city and Young Spock apologizes to his parents, saying that he needed to take the test on his own and that he now realizes he is ready to choose the Vulcan way. He then goes to demonstrate the newly learned Vulcan neck pinch on the bullies. Sarek thanks Spock for his help and says he owes him everything. Spock asks that he merely try to understand Young Spock and departs.
Spock returns to the present and beams up to the Enterprise
with Kirk. Kirk asks what happened and notes that the death of a pet wouldn't be that significant in the course of history. Spock wonders at that. McCoy recognizes him and starts complaining about having to recalibrate his medical instruments, and Spock notes that he would have a much more difficult time of it calibrating his instruments for an Andorian.