Stardate 5373.4: When Kirk, Spock and the historian Erikson return from a mission to Orion's past using the Guardian of Forever, no one else recognizes Mr. Spock; the ship's first officer for the past five years has been an Andorian named Thelin. It turns out that the son of Sarek and Amanda died on Vulcan when seven years old. Spock recalls that during the Vulcan ritual of kahs-wan a cousin named Selek saved his life, a man who must have been the older Spock himself. Spock returns to his childhood to fulfill Selek's duty. In the process young Spock's pet sehlat I-Chaya is mortally injured. The boy chooses to compassionately end his pet's suffering and in doing so sets the young half-human, half-Vulcan on a path toward embracing Vulcan traditions. Upon returning to the present, Spock finds the timeline and his place on the USS Enterprise restored.

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