Following a voyage back in time to observe the history of the Orion civilization, a team consisting of Kirk, Spock, and an historian named Erikson return to the year 2269 from the enigmatic time portal known as the Guardian of Forever — only to discover that no one other than Kirk and Erikson knows Spock, whose job as the Enterprise's second in command is now inexplicably being filled by an Andorian named Thelin. Reasoning that history must have somehow become disrupted during the team's trip through the Guardian, Spock consults the records stored in the starship's library computer, which reveal that he died on Vulcan at the age of seven; because the change to history occurred while Spock was in the past, his adult self has apparently avoided abruptly winking out of existence.

Spock recalls that at the age of seven he did indeed nearly die at the time of his kahs-wan test, the traditional desert survival rite that marks a young Vulcan's coming of age, only to be rescued by a cousin named Selek, whom Spock never saw again afterward. To restore the integrity of the timeline — and to recover his rightful place in it — Spock uses the Guardian to return to the planet Vulcan of more than three decades earlier, where he poses as Selek, the very same cousin he remembers as having saved his life. Faced with having to choose between his logical Vulcan and emotional human heritages, the young Spock impulsively heads out into the desert of Vulcan's Forge to grapple with the question, trailed by his elderly pet sehlat I-Chaya. In the desert, a fierce predator known as a le-matya, attacks, forcing I-Chaya to fight for young Spock's life; fortunately, "cousin Selek? intervenes, stopping the le-matya with a neck pinch, but not before the creature's poisonous claws strike I-Chaya.

Young Spock urgently summons a healer, who arrives too late to save I-Chaya's life; all he can do is keep Spock's beloved pet alive and in pain. Spock makes the logical, if painful, decision to release the mortally wounded I-Chaya from life; the creature dies, though this didn't occur in Spock's original timeline. After saying farewell to his father Sarek — and asking him to try to understand his son in future years — the adult Spock returns to the present, where he finds the timeline, and his life, restored to normal, except for the death of a fondly remembered childhood pet.