Yesterday ' s Son " Spock discovers his own son – living 5 , 000 years in the past ! " Yesterday ' s Son is a Pocket TOS novel – # 11 in the numbered series , and the first book in the retroactively designated " Yesterday Saga " – written by A . C . Crispin . Published by Pocket Books , it was first released in . Summary . While studying the archaeological records of the now - destroyed planet Sarpeidon , a scholar aboard the " Enterprise " finds pictures of an ice age cave painting depicting a Vulcan face . After bringing this to Spock ' s attention , Spock realizes that his involvement with Zarabeth when he and McCoy were accidentally stranded in the planet ' s ice age has resulted in the birth of a child . Spock asks Captain Kirk for shore leave and after Kirk insists that Spock tell him why , Spock admits what happened in the past and that he must go to and speak to T ' Pau and ask her to petition the Federation Council for permission to use the Guardian of Forever to travel back to the past and retrieve his son . T ' Pau agrees to intercede on Spock ' s behalf and Spock is granted permission to use the Guardian . Kirk , however has required from Spock that he be able to accompany Spock on his journey . On their way , they are joined by Dr . McCoy , who also intends to go , seeing as how he also remembers Zarabeth and has been working on and subsequently discovered a way to medically reverse Zarabeth ' s Atavachron conditioning so that she can return to the future along with them . Spock tells Kirk that he anticipated McCoy ' s desire to accompany them and so he asked T ' Pau to secure permission for the three of them . Using the Guardian , they journey back into Sarpeidon ' s past and rescue his son . However , due to a miscalculation on Spock ' s part , they find a young man of twenty - eight instead of a small child , who tells them that his name is Zar and that his mother Zarabeth died in an accident many years before . Spock introduces himself but refuses to allow Zar to call him " Father . " Zar shows Spock where Zarabeth is buried and Spock uses his phaser to cremate Zarabeth ' s remains . Zar returns to the " Enterprise " and passes as a distant relative of Spock , who oversees his education and attempts to train him in Vulcan telepathic techniques . They discover that Zar is an unusually strong telepath for a Vulcan ; he can establish contact without touching the other person . Zar becomes conflicted and hurt by his father ' s apparent refusal to acknowledge him . The " Enterprise " is called back to Gateway to protect the Guardian of Forever from a Romulan intelligence raid . It is imperative to the security of the Federation that the Romulans not discover the Guardian ' s powers ; if they cannot be driven away , Gateway must be destroyed . The Romulans place a force field around the Guardian and hide themselves behind a ground - based cloaking device . Zar volunteers to help Spock counteract the force field because he can sense whether Romulans are present even if he cannot see them . Their first try is unsuccessful , but when they rendezvous with Kirk the three men discover they are trapped on the planet ; the " Enterprise , " with Scotty in command , is battling the Romulans ( along with the ) and cannot drop its shields to beam them up . They decide to try again , but Spock disables Zar with the Vulcan nerve pinch , wishing to spare him from danger . Kirk and Spock are captured and tortured by the Romulans . When Zar wakes up , he is able to telepathically sense their danger . He also realizes that his father cares about him , since he chose to protect Zar instead of Kirk , his closest friend . The " Enterprise " defeats the Romulan ships and a rescue party beams down . Zar creates a diversion by causing an explosion , allowing the others to rescue Kirk and Spock . Once the Romulans threat is over , Zar decides to use the Guardian to return to Sarpeidon ' s past , but to a more settled location than the one he originally inhabited . Although Spock offers him a place among his family on Vulcan , there is evidence that Zar ' s return is crucial to Sarpeidon ' s unusually rapid cultural evolution . After a brief family mind meld with Spock , Zar enters the time portal . Memorable quotes . " " As I sit in the solitude of my cabin , I know I must begin considering precisely what I should do about the face on the wall of the cave . . " " " " That is tantamount to blackmail , Jim . " " " " Not tantamount , it " is " blackmail . " " " " Taking in account Doctor McCoy ' s predictable penchant for rushing in where angels fear to tread , I reasoned he would attempt this . There ' s usually a logical pattern to the good doctor ' s illogical pattern . " " " " Why you pointed eared , ungrateful – " " " " Next stop , Gateway . And the Guardian of Forever . " " " " I ' ve got to hand it to you Spock , what a great place to spend our leave . Beautiful warm sunlight , soft breezes , the natives are friend – wha – what was that ? " " " " Probably a Sethar , Bones . A huge , very savage predator . Their favorite meal is chief surgeons , who don ' t have the sense to follow orders given by their commanding officers . " " " " Looks like you miscalculated a bit , Spock . But better late than never , right ? " " " " I would prefer you address me by my name . I find the appellation " father " inappropriate , when used by a . . . stranger . " " " " I ' m sorry – I didn ' t realize – I was only trying to - " " " " On Vulcan , what you attempted just now is regarded as a heinous crime . Forcing a meld , is an unforgivable intrusion of the spirit . You must never do so again . Do you understand ? " " " " Yes sir , I acted on impulse . It was wrong . I know better now , I won ' t ever forget . " " " " See that you do not , or I will not be able to continue your training . " " " " I suppose you could call it training , as if I was some stray animal you rescued . But I think it ' s much closer to the programming you do to your computers . I can ' t touch you ! Why ! ? " " " " The simple fact of my existence mattered to him . I don ' t . There ' s only one person Commander Spock cares deeply for , and that ' s . . . Not me . " "