An unidentified signal draws the Enterprise off course to a distant planet on the edge of the galaxy. Spock determines that the planet is uninhabitable due to some ancient cataclysm. They’re interrupted when a voice mentally speaks to them, identifying itself as Sargon. It refers to them as his “children” and asks that they approach the planet below.
I am Sargon. It is the energy of my thoughts which has touched your instruments and directed you here.
Kirk orders the ship close enough to the planet to reestablish contact. Sargon asks that they beam down and he will explain to them. Spock detects a chamber a hundred miles into the planet capable of maintaining life. Sargon assures them that he can use the ship’s transporters to beam them there unharmed. Kirk suggests that Spock stay behind, but Sargon causes the power levels to drop until the captain relents. They arrive in the transporter room to find Lt. Anne Mulhall, ship’s astrophysicist, waiting for them. She says she too was “ordered” to join the landing party, although she can’t say exactly how. McCoy is skeptical of the whole thing but agrees to go, as will two security guards. However, Sargon takes control of the transporter and only beams down Kirk, Mulhall, Spock, and McCoy.
Well, I'm not a liar, Captain. I did receive an order to report here for duty.
I'm sure she did, Captain, just as you received an order to bring me along.
The landing party arrives in an underground chamber and finds a glowing sphere. Sargon’s voice speaks from it, explaining that 500,000 years ago he and the others of his race fought a great war and almost wiped themselves out. He explains that their mental powers grew so great that they came to believe themselves gods and fought among each other for supremacy. When Kirk wants to know what Sargon wants, Sargon transfers his consciousness into Kirk’s body, and Kirk’s into the globe. Sargon then takes the others to the next chamber which is filled with the globe receptacles. However, all but two have gone dead. One of them is Thalassa, Sargon’s wife. The other is Henoch who was on the other side of the conflict until the end when they all worked together to preserve what was left of their race.
Where's our captain? Where's Jim Kirk?
He is... unharmed. I ... have taken his body to demonstrate.
Sargon explains that the three of them want to borrow the bodies of Kirk, Mulhall, and Spock long enough to make android bodies that they can permanently occupy and then pass on their knowledge to their “children.” McCoy warns that Sargon’s presence is accelerating Kirk’s metabolism to lethal levels. A weakened Sargon returns to his sphere and Kirk admits he is impressed with what he shared with Sargon as their minds passed during the exchange.
When Sargon and I exchanged, as we passed each other, 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.
Aboard the ship, Kirk calls a meeting and insists that they go ahead over the objections of McCoy and Scotty, because “risk is their business.” They beam aboard the spheres and the transfer goes as plan. However, Sargon and Thalassa quickly weaken while Henoch, thanks to Spock’s physically superior body, is still able to function. Sargon has him prepare a metabolic reduction serum so that they can stay in the bodies safely for a longer period of time. Henoch agrees and goes to work with Nurse Chapel. However, he creates a special lethal formula for Sargon. When Chapel realizes something is wrong, he mentally dominates her then orders her to give the injections.
Is something wrong, Miss Chapel?
Uh... yes. I, uh...
I had... something to say. I... can't seem to remember.
Sargon and Thalassa begin work on the android bodies while remembering 500,000 years ago when they shared physical bodies. Sargon weakens more quickly while Henoch taunt Thalassa with the idea of spending eternity in an unfeeling android body. She begins to agree with Henoch that they should keep the bodies, but goes to Sargon who collapses. McCoy pronounces him dead and puts him on life support to maintain his physical function. Thalassa secretly offers McCoy the chance to have Kirk back alive and whole, but demands that she keep Mulhall’s body. McCoy refuses and Thalassa briefly tortures him with her mental powers until she realizes she has grown corrupted. Sargon then speaks out praising her, and they realize that Sargon has stored his consciousness in the ship itself. He summons Chapel in and has McCoy to leave. The ship shakes and when McCoy returns, he finds that Kirk and Mulhall are back in their bodies and the three spheres have been destroyed. Sargon and Thalassa are dead, as is Spock’s consciousness.
I'm a physician.
A physician? In contrast to what we are, you are a prancing, savage medicine man. You dare defy one you should be on your knees worshipping? I could destroy you with a single thought.
Kirk orders McCoy to prepare the most potent Vulcan poison he has, and they go to the bridge where Henoch is torturing the crew. Henoch freezes Kirk and Mulhall with pain then immobilizes McCoy and tells the still controlled Chapel to inject McCoy with it. However, Chapel injects Henoch with it. He tries to leave Spock’s body but realizes that Sargon is still alive and forcing him to die with the host. Henoch dies and Sargon explains that he couldn’t let Kirk’s friend die. The “poison” was a simple knockout drug that McCoy was lead to believe was poison, so that Henoch would read it in his mind and believe it as well. Sargon stored Spock’s consciousness in Nurse Chapel and then restores it to the proper body. Sargon realizes that he and his wife could never withstand the temptation and ask to borrow Mulhall’s and Kirk’s body for one last kiss before departing into oblivion.
Oblivion together does not frighten me, beloved. Promise we'll be together.