Yesteryear ( episode ) Spock travels back in time to prevent his own demise during his youth on Vulcan . Summary . Act one . As Federation historians Aleek - Om and Grey as well as CMO McCoy record history , while standing outside the Guardian of Forever , the Guardian announces the return of the travelers Kirk , Spock and historian Erickson through its gateway , after a mission to observe the dawn of the civilization . As the travelers begin to appear , one - by - one , Kirk announces to McCoy how exciting it was to observe the birth of a civilization , just as the last traveler , Spock , emerges from the Guardian , much to the surprise of both Grey and McCoy . McCoy inquires who it is they brought back with them . Although Kirk reacts with puzzlement because he believes McCoy obviously knows Spock , McCoy says he doesn ' t . The crew members beam back to the ship and are greeted by Scotty . He was not expecting a Vulcan among those he transported back , and was anticipating one of the historians instead . Annoyed , Kirk responds that he expects his first officer to be treated with respect , just as an Andorian crew member enters the transporter room , stating that no - one has ever treated him otherwise . Kirk inquires as to who the newcomer is and McCoy then introduces – or rather " re " - introduces – Commander Thelin , making an attempt to remind Kirk that Thelin has been his first officer aboard the " Enterprise " for the past five years . Spock and Kirk finally come to the realization that what they have encountered is not a game , and question what is going on . In a staff meeting in the briefing room , Lieutenant Erickson reviews the tricorder logs from their mission , and observes that there is nothing they could have possibly done to change the events of the future . Spock then surmises that the change in the timeline must have happened while they were in Orion ' s past . The meeting is then interrupted by Bates , who just checked Starfleet records on Spock , as requested by Thelin . Bates reports there is no Vulcan named Spock serving with the Starfleet in any capacity . Thelin then inquires about the results of the Vulcan family history request . Bates displays an image of Sarek of , and notes that he has been an ambassador to seventeen Federation planets in the past thirty year s . Spock notes that this information is incorrect and inquires about Sarek ' s family – his wife and son . Bates then transmits an image of Amanda Grayson , the former wife of Sarek , from whom she separated following the death of their son . Amanda was later killed in a shuttle accident at Lunaport on her way home to Earth . Sarek has not remarried . Spock mourns briefly for his mother , then inquires as to the name of the son that died ; it is Spock , aged seven upon his demise . Kirk , Spock and Thelin return to the surface of the time planet to confer with Aleek - Om and Grey on what happened . Kirk asks if the Guardian was in use while they were away , and Grey informs him that it was used , in only in a limited capacity , to scan recent Vulcan history , twenty to thirty Vulcan years past . Kirk then wonders if there is any notation on the death of Sarek ' s son , and Aleek - Om confirms that there was , and that he had died during the " kahs - wan " maturity test . Spock then recalls the date of the event in question , the twentieth day of Tasmeen , noting that it was the day that his cousin Selek saved his life in the desert when he was attacked by a wild animal . Although Spock cannot remember the finer details of the event , he recalls that that was also the only time he had ever met Selek . On a hunch , Kirk asks if Selek looked anything like how Spock now does . Spock confirms what Kirk has been thinking ; Spock saved his own life in the original timeline , but was unable to do it a second time because he was in Orion ' s past when the time vortex replayed Vulcan history , making it impossible for him to be in two places at one time . With the realization of what has transpired , Kirk asks the Guardian if there is any way they can reverse what happened . The Guardian confirms that there is , as long as no other important factor is altered . Spock decides that , in order for him to save the lives of both his mother and himself , he has to return to Vulcan and correct what has been changed . Before his departure , he requests a Vulcan desert soft - suit and boots , as well as a small selection of street wear and carry bag , circa 8877 Vulcan year , from the ship ' s wardrobe section . Kirk flips open his communicator so it can be done . A brief conversation with Thelin reveals that he bodes no ill will towards Spock ' s quest , despite the fact that this quest will change the commander ' s timeline as well . He considers it a reasonable sacrifice in order to save Spock ' s family . Likewise , Spock wishes Thelin a long and prosperous life , in whatever circumstances the hopefully repaired timeline will put him in . Spock hurriedly enters the Guardian and travels back to his hometown , ShiKahr , in the month of Tasmeen , thirty years in the past , in 2237 . Act two . Spock meets his father , Sarek , as well as his younger self ( who is being harassed by three young Vulcan bullies – Sepek , Sofek and Stark ) , and assumes the identity of cousin Selek , introducing himself to Sarek in such a manner . Young Spock goes on his " kahs - wan " a month early , on his own will . His pet " sehlat " , I - Chaya , follows him into the Vulcan desert , defying young Spock ' s authority . In the meantime , adult Spock / Selek , about to turn in for the night , suddenly realizes and says aloud , " " Of course , I should have remembered ! It " wasn ' t " the actual " kahs - wan " ordeal . " " Upon that realization , he rises again and dresses , following young Spock and I - Chaya into the desert . In the desert , the pair are attacked by a ferocious " le - matya " . Adult Spock , who had been following the two into the desert , intervenes and saves young Spock ' s life , incapacitating the large creature with a Vulcan nerve pinch – but he is too late . For unlike the way adult Spock remembers this episode , I - Chaya is badly injured from the battle and is dying . Act three . Young Spock sets out to find a healer in the city . Meanwhile , adult Spock uses a nerve pinch to help ease I - Chaya ' s pain , telling the creature that he previously undertook the " kahs - wan " ritual without I - Chaya having to be sacrificed . Although young Spock is successful , I - Chaya is too far gone by the time the healer arrives . The healer gives young Spock the choice to give I - Chaya a longer but painful life , or to release him from his suffering . Spock chooses to release I - Chaya from life , the most logical way , and , in doing so , chooses to follow the Vulcan ways of his father . The timeline is saved and Spock returns from the Guardian as first officer . Before beaming back to the " Enterprise " , Spock laments the death of I - Chaya to Kirk , admitting that this did not happen the first time around . Kirk , however , cannot understand how a pet could mean so much in the course of time . In the " Enterprise " s transporter room , McCoy is complaining when it is Spock ' s turn to take a routine physical , and bemoans that he has to recalibrate his medical scanner for a Vulcan every time . Spock tells him that if things were different , he would be recalibrating for an Andorian instead . McCoy asks if this is some sort of joke , reminding Spock that Vulcans don ' t tell jokes . To this , Spock responds by pointing out to the doctor that " times change . " " Memorable quotes . " " Is it possible for Spock to return to Vulcan and repair the timeline that has been broken so all is the same as before ? " " " " It is possible if no other major factor is changed . " " " " What a trip , Bones ! " " " " In the family , all is silence . No more will be said of it . Live long and prosper , Sarek of Vulcan . " " " " Who are you ? " " " " Oh , I ' m sure you ' d know Thelin by now , Jim . He ' s been your first officer for five years . " " " " You will not disappoint me . Not if your heart and spirit " are " Vulcan . " " " " What you do not yet understand , Spock , is that Vulcans do not lack emotion . It is only that ours is controlled . Logic offers a serenity Humans seldom experience in full . We have emotions , but we deal with them . . . and do not let them control us . " " " " Earther ! Barbarian ! Emotional Earther ! You ' re a Terran , Spock ! You could never be a true Vulcan ! " " " " That is not true ! My father . . . " " " " Your father brought shame to Vulcan . He married a Human ! You haven ' t even mastered a simple Vulcan neck pinch yet , Earther ! " " " " What ' s the matter , Bones ? " " " " Who ' s he , Jim ? " " " " Whadda you mean , ' Who ' s he ' ? You know Mr . Spock . " " " " ' Fraid I don ' t , Jim . " " " " One small thing was changed this time . A pet . . . died . " " " " A pet ? Well , that wouldn ' t mean much in the course of time . " " " " It might , to some . " " " " Every life comes to an end when time demands it . Loss of life is to be mourned but only if the life was wasted . I - Chaya ' s was " not . " " " Doctor McCoy , you do not know your good fortune . If the times were different , you would have to recalibrate for an Andorian . " " " " What ' s that supposed to mean ? If that was supposed to be a joke , Spock , I have to remind you Vulcans don ' t tell jokes . " " " " Times change , doctor . Times change . " " Links and references . References . 2230 ; 2237 ; 2264 ; 8877 ; adulthood ; Andorian ; annual physical ; Aurelian s ; " Bones " ; boot s ; carry bag ; cousin ; death ; desert flyer ; Earth ; Earther ; emotion ; Federation ; Federation history ; forefather ; healer ; historian ; joke ; " kahs - wan " ; L - langon Mountains ; " le - matya " ; logic ; Lunaport ; medical scanner ; Milky Way Galaxy ; ; Orion s ; pet ; philosophy ; physical ; poison ; practical joke ; Sasak ; " sehlat " ; Selek ; separation ; ShiKahr ; street s ; sucker vine ; ; Tasmeen ; tear ; Terran ; timeline ; time planet ; vacation ; ; Vulcan s ; Vulcan desert soft - suit ; Vulcan ' s Forge ; Vulcan history ; Vulcan neck pinch ; wardrobe section