Return to Tomorrow ( episode ) Three survivors from a race that died half a million years ago " borrow " the bodies of " Enterprise " crew members so they can build android bodies for themselves . Summary . Teaser . The is traveling through a region of space hundreds of light year s farther than any Earth starship has ever explored . A great , ineffable intelligence has activated her distress signal relays , giving her strong readings yet remaining invisible to her sensors . The crew arrive at a destroyed class M planet – much older than Earth , Spock determines , and long dead , its atmosphere ripped away by some cataclysmic event about half a million years ago . A male voice suddenly speaks , referring to the ship ' s crew as his " children " and asking them to come into orbit . He admits the unpromising state of his planet , and says strangely that he too is dead – and death will be the fate of mankind too , should they choose not to visit . Act One . In his captain ' s log ( Stardate 4768 . 3 – " see below " ) , Captain Kirk states his intention to risk contact ; Lieutenant Uhura tells him that the entry will not reach Starfleet for three weeks due to the " Enterprise " s distance from known space . Spock ' s science station probe s touch the mysterious planetary speaker , named Sargon , who feeds him the transporter coordinates to a chamber more than a hundred miles beneath the surface . In that deep vastness , Spock detects a serviceable atmosphere and presumes that a landing party should fare well enough . Kirk plans to leave him in command , saying that with this many unknowns " " we can ' t risk both of us being off of the ship . " " But Sargon makes his preferences plain by cutting the ship ' s power completely until Spock is added to the guest list . Kirk now asks Spock to accompany him and leaves Lieutenant Sulu in command of the " Enterprise " . In the transporter room , Dr . McCoy , crewmen Lemli and Leslie , and Lt . Commander Mulhall have reported for beam - down . Mulhall , an astrobiologist seconded to the operations division , is unknown to Kirk ; it turns out that her orders to join the landing party came from Sargon himself . McCoy is apoplectic when he hears Spock ' s revised approximation of the thickness of solid rock through which the party is to be transported : 112 . 37 miles . When the party leaves the ship , the two crewmen ' s transporter pads fail to energize – another one of Sargon ' s surprises . Deep underground , the " Enterprise " landing party materializes in a sort of holding area . Spock finds that the walls date from the time of the cataclysm , and are made from the strongest , hardest material he has ever come across . Mulhall finds the atmosphere only slightly different from that aboard ship . A chamber opens and the unguarded party enter to discover Sargon – energy without substance , matter without form – housed in a glowing spheroid shell . He gives his guests a little history lesson : 6 , 000 centuries ago , the humanoid s of this planet were spacefarers . They colonized throughout the galaxy . Sargon speculates that a Human creation myth were perhaps two beings of Sargon ' s race . Mulhall objects to this idea , but Spock picks up on it , saying it might explain away some elements of Vulcan pre - history . But 1 , 000 centuries after the colonial heyday came the ultimate conflict . Possessed of minds " infinitely greater " than the landing party ' s , having goals beyond their comprehension , Sargon ' s race fought a superwar , unleashing powers to which even nuclear war pales in comparison . And so the masters of the galaxy all but exterminated themselves , and their homeworld for half a million years has lain dead . Calling Kirk his " son " , Sargon exchanges places with him , taking the captain ' s body from him and storing his mute mind within the sphere . Sargon is thrilled to have a corporeal form again , and states his intention of using Kirk ' s , Spock ' s and Mulhall ' s bodies . Act Two . Leading the landing party to an inner chamber , Sargon shows them ten other spheres ranged in two rows . His wife Thalassa ' s is the only one still aglow on the lower tier . On the upper , one glows as well – Henoch , of the ultimate conflict ' s " other side . " These essences , too , will require hosts , namely Mulhall and Spock . McCoy complains that Sargon is " burning up " Kirk ' s body – his heart is beating 262 times per minute . Sargon says he and his fellows wish to hold the Human and Vulcan bodies only long enough to build " humanoid robots " with methods and skills " far beyond your abilities . " Sargon staggers back to the main chamber and , again calling Kirk " son , " vacates his body . Kirk ' s metabolism promptly returns to normal . He says his mind ' s stay in the receptacle was a " floating in time and space . " But the intimate proximity to Sargon during the exchange has affected him deeply : " " For an instant we were one . . . I know him now . I know what he is and what he wants – and I don ' t fear him . " " In a briefing room back aboard the " Enterprise " , the landing party is joined by Scott , whose assistance will be necessary in the construction of " android robots . " Kirk will not order their participation . With such mechanical bodies , Spock says , Sargon and company will be able to leave this planet and share their technology . The resulting advances for " mankind " would be a great leap of ten millennia . Scott is won over by the prospect of starship engines being " the size of walnuts " ; Mulhall says that in the interest of science she must cooperate . McCoy finds it a suspicious coincidence that the bodies of both the captain and first officer are required for the task ahead and worries that to such " giants " the Human crew must be " insects . " Kirk compares the undertaking at hand with the first Earth missions to the Moon , to Mars , to Alpha Centauri , and reminds McCoy that six generations ago surgery was done with scalpels and catgut . " " Risk . . . " " he concludes , " " risk is our business . That ' s what this starship is all about . That ' s " why " we ' re aboard her . " " Spock , McCoy , Scott , and Mulhall ' s doubts about the transference are erased after Kirk ' s passionate speech . In the medical lab , McCoy and Nurse Chapel oversee the transference . Thalassa , seeing through Mulhall ' s eyes , at first looks for her husband in Spock , but Sargon draws her attention " here " – in Kirk ' s body . She approves of his choice of host , finding it similar to the body he lost in the cataclysm . Henoch is pleased with his own host : the Human - Vulcan hybrid has " strength , hearing and eyesight , all far above your Human norms . " Spock ' s body deals better with the transference too , being " accustomed to the higher metabolism " ; Henoch stays in it when the others collapse and for the second time Sargon relinquishes Kirk ' s body . In the pharmacology laboratory , Henoch and Chapel make up a metabolic reduction formula which , administered from a hypospray at 10 cc / hr , should allow the three cataclysm survivors to function in their host bodies . Chapel notices that Sargon ' s formula has been doctored and fears for her captain . Henoch confirms that he intends that Kirk die – so as to finish off Sargon . He then establishes mind control over Chapel , touching his middle finger to her forehead , making her forget about what he just said . Act Three . McCoy makes an entry in his medical log ( 4769 . 1 – " see below " ) : Sargon is now in his third possession of Kirk ' s body , Thalassa is back in Mulhall ' s , and Henoch continues to possess Spock ' s . In a science laboratory , Sargon and Thalassa are beginning the assembly of their new android bodies . They reminisce , but the lost scenes of their beloved homeworld turn to a cruel reminder of the insensate future that awaits them . Henoch enters and enjoys the plight of his sorely tempted but morally rigorous opponents . He for one has no intention of relinquishing his host body . Sargon feels the damage he is wreaking on Kirk ' s inadequately suppressed metabolism , but does not want to worry his wife and soldiers on . In the medical lab , Chapel conveys Henoch ' s bogus metabolic readings to McCoy . Mistaking her evident stress for fatigue , he offers to administer the last few doses of formula to the visitors . Alarmed , Chapel insists she will be up to the task . In the shop , Scott cannot see how the technology of the ancient colonizers is going to work in the android bodies , which must " need micro - gears and some sort of pulley that does what a muscle does . " A happy Henoch / Spock appears in the doorway and corrects him : comprehend its workings though Scott may not , the android form will surpass his Human strength and agility by 100 % . To Thalassa , though , Henoch ' s upbeat manner rings hollow : the android form will house her for a thousand years – is that not what a prison does ? She feels a tremor of revolt against the impending surrender of her humanoid existence . Might a Human body not after all be her due , given all the good she proposes to bring to mankind ? In the deck six briefing room , Sargon realizes he can no longer ignore the danger to Kirk ' s body and calls McCoy . Thalassa arrives first and runs the idea of sacrificing Mulhall past her husband . Seeing that the wrongness of the proposition has escaped her , Sargon points out the practical difficulties – it will take months , if not years , for the host bodies to grow accustomed to the presence of their essences . She knows that he , too , longs to resume their physical intimacy – the " intertwining " of senses – and kisses him , asking " can robot lips do this ? " Fighting temptation as well as Henoch ' s vapid medicine , he collapses . McCoy and Chapel arrive to find Sargon / Kirk " dead . " Act Four . McCoy makes an entry in the medical log ( 4770 . 3 – " see below " ) . He is sure that Sargon has died , forced to flee the captain ' s collapsed body , and too far from the receptacle to bring about the exchange of essences . Despite its inhospitality , the vacant body has been brought around in sickbay , its " vital organs now working , " as the nurse says . In the shop , Henoch works on a male - shaped android body . Thalassa wonders why he bothers , since he clearly doesn ' t plan to return his present body to Spock . He stokes her revulsion , saying the android form is for her – she can occupy it before it has its female features installed . She cannot bring herself to put her consciousness into the android body . In the medical lab , Thalassa proposes a back room deal with McCoy : she is able to move Kirk ' s mind back into the functioning body , but she requires that the doctor connive at her keeping that of Mulhall , " whom you hardly know – almost a stranger to you ! " Even for such a return , McCoy cannot leave Mulhall to die . Thalassa threatens him , " we can take what we wish . . . I could destroy you with a single thought ! " She projects fire onto McCoy , but soon her godlike posturing disgusts her , and she realizes that physical existence is seductive and corrupting for her kind . Relieved to witness her integrity , Sargon now speaks : he has been sheltering unsuspected in the very fabric of the vessel . Chapel arrives summoned . Thalassa asks McCoy to leave them stating – " Sargon has a plan . " In his study , McCoy is alarmed by a series of explosions from the lab . Unable to enter , he is calling for aid when Chapel exits – with something plainly on her mind . Back in the lab , Kirk and Mulhall have regained their bodies . The three receptacles are burnt out – completely destroyed . And Spock ' s mind ? Kirk says the loss was " necessary . " The urgent task is to terminate Spock ' s physical form and so put an end to Henoch . Kirk orders McCoy to prepare a hypo fatal to Vulcans . On the bridge , Henoch terrorizes Uhura , who screams . Henoch sits in Kirk ' s chair and warns helmsman Sulu not to fight him . Chapel stands beside Henoch . McCoy arrives with Kirk and Mulhall , whom Henoch stops short by forcing pain upon them near the turbolift . Chapel takes McCoy ' s hypo . Its contents are known to Henoch from reading McCoy ' s mind . He orders Chapel to inject McCoy – but suddenly , she injects him / Spock instead . At first he belittles their attempt , but when he senses the undead and powerful Sargon his instinct is to beg . Spock ' s body collapses , Henoch flees but with no host , android nor receptacle at hand he is destroyed . Chapel swoons , Spock stands . She explains that she had been carrying ( and sharing ) Spock ' s essence ( or katra ) , which behind Henoch ' s back Sargon had " placed in me . " And the hypo , potent enough to " kill ten Vulcans " ? A necessary illusion , says Sargon . He borrows Kirk ' s body for a fourth and last time , in order to hold his wife as a living woman before the couple consign themselves to oblivion " forever . " Their final kiss , says Chapel with a teary , fond look at Spock , was " beautiful . " The " Enterprise " leaves Arret and continues its exploration of the galaxy . Memorable quotes . " " One day our minds became so powerful , we dared think of ourselves as gods . " " " " We must have Captain Kirk and you – so that we may live again . " " " " They used to say if man could fly , he ' d have wings . . . but he did fly . He discovered he had to . Do you wish that the first Apollo mission hadn ' t reached the moon , or that we hadn ' t gone on to Mars or the nearest star ? That ' s like saying you wish that you still operated with scalpels and sewed your patients up with catgut like your great - great - great - great - grandfather used to . I ' m in command . I could order this . But I ' m not . . . because . . . Dr . McCoy is right in pointing out the enormous danger potential in any contact with life and intelligence as fantastically advanced as this . But I must point out that the possibilities , the potential for knowledge and advancement is equally great . Risk . . . risk is our business ! That ' s what this starship is all about . . . that ' s why we ' re aboard her ! " " " " Oh , you are a lovely female . A pleasant sight to wake up to after half a million years . " " " " Thank you . " " " " I ' m surprised the Vulcans never conquered your race . " " " " Vulcans worship peace above all , Henoch . " " " " I will not peddle flesh . I ' m a physician . " " " " A physician ? In contrast to what we are , you are a prancing , savage medicine man . " " " " Spock ' s consciousness is gone . We must kill his body – the " thing " in it . " " " " Oblivion together does not frighten me , beloved . Promise we ' ll be together . " " " " I promise , beloved . " " Background information . Remastered information . The remastered version of this episode aired in many North American markets during the weekend of . It featured new effects shots of the " Enterprise " and a new , more realistic version of Sargon ' s homeworld . It also included shots of the planet matted into interior viewscreen shots . Links and references . References . Adam and Eve ; air ; Alpha Centauri ; Apollo 11 ; Arret ; artery ; astrobiology ; atmosphere ; blood ; body temperature ; " Bones " ; catgut ; cc ; class M ; conn ; diagram ; distress signal relay ; Earth ; energy ; engineer ; euphoria ; evolution ; Fahrenheit ; formula ; great - great - great - great - grandfather ; heart ; heart rate ; humanoid robot ; hypo ; injection ; jelly ; landing party ; lung s ; Mars ; matter ; medicine man ; metabolic rate ; metabolic reduction injection ; micro - gear ; mile ; Milky Way Galaxy ; muscle ; negatron hydrocoil ; nitrogen ; nuclear age ; oxygen ; pharmacology ; physician ; prejudice ; pulley ; receptacle ; Sargon ; Sargon ' s species ; scalpel ; standard orbit ; subspace radio ; technician ; transporter ; tricorder ; vital organ ; Vulcan ; Vulcan prehistory ; walnut