Plot

The Enterprise is guided to a distant, long-dead world where survivors of an extremely ancient race - existing only as disembodied energy - desiring the bodies of Kirk, Spock and astro-biologist Ann Mulhall so that they may live again.

From a planet bereft of life for half a million years, the Enterprise hears the voice of Sargon, who is able to control the ship and tells them to transport to specific coordinates which target them to a subterranean chamber. The away party consisted of Kirk, Spock, McCoy and astro-biologist Ann Mulhall; the security guards they planned to take along were prevented from de-materializing. Sargon is one of only three survivors of the planet's intelligent race - pure energy, matter without form. They tell the away party that they once started life on Earth and many other places. Suddenly Sargon possesses Kirk's body, saying he requires Spock and Ann Mulhall's bodies, too, in order to give the only other survivors of his race new life. He promises the bodies will be returned after they build superior mechanical robots as their definitive bodies, then leaves Kirk's and allows them to beam up and freely make up their minds. McCoy isn't tempted by curiosity and potential benefits, but Kirk convinces him to re-think his veto. However the human bodies need a metabolic reduction injection; the alien in Spock's body uses Vulcan mind control to make nurse Christine Chapel give Kirk, Sargon's host, an injection that would kill Sargon (and also Kirk). Sargon's wife implores Sargon to leave Kirk's body before they both die - can Kirk be revived, as his mind is still in Sargon's mind-receptacle from the planet. The aliens have more surprises in store.—KGF Vissers

Brought deep into an uncharted part of the galaxy, the Enterprise comes across three disembodied beings, their essence each contained in a globe-like receptacle. Their leader, Sargon, asks only one thing of Kirk and his crew - lend them the bodies of Kirk, Spock and crew member Ann Mulhall long enough for them to build robot bodies to inhabit for perpetuity. The beings have been without physical form since their civilization was destroyed over 500,000 years ago but have powers far greater than ordinary humans. Kirk and the rest agree to the exchange, but the alien occupying Spock's body, Henoch, clearly has designs on keeping the body he has just obtained. When he manages to convince Thalassa to do the same, Sargon - and the body of Capt. Kirk - is in trouble.—garykmcd

Anne Mulhall (Diana Muldaur) is a red-haired Irish crew member with special skills in astrobiology

The Enterprise has been summoned by a powerful force named Sargon. He explains that he needs to speak to Captain Kirk, Anne Mulhall and Mr Spock immediately. Spock says that the planet has been devoid of life for .5 MM yrs after a cataclysmic event. When they beam down, they find themselves in a chamber 1000 meters below the surface. There is a glowing light inside a plastic pillar bubble. This is Sargon. Sargon has no body. He is the mind of a powerful being from centuries before, his mind preserved in the bubble. His voice explains to Kirk. "We were a powerful race, but as our power grew, we became envious, and destroyed each other. Sargon says that eons ago their ancestors explored the galaxy as humans do now and hence it is possible that humans are their descendants. The greatest minds from both sides of Sargon's war were preserved in these bubbles." Kirk sees many bubbles in the chamber, but most are dark. Sargon says "We knew our mistakes. We need to share what happened with the universe, and prevent it from happening to others. Only three of us have survived. Myself, my wife Thalassa, and Hennock, one of our enemies. Now we must work together, but we cannot, inside these bubbles. We need to have human form, so that we may create robot bodies for ourselves that will contain our essences." The possession of the body by Sargon increases the metabolic rate and the heart rate, and thus Sargon can occupy it only for a short while.

Kirk is told that it will only be done with his consent. After a debate with his officers, Kirk explains that the mission of the Enterprise is to contact new life and to be open. He consents, and after a brief possession of his body by Sargon, agrees to allow the three to possess three of them, with their consent. Thalassa possesses Anne Mulhall, and Hennock possesses Spock. Aboard the ship, Kirk, Spock and Anne's consciousnesses are in the bubbles, and the super-beings begin making the robot bodies. But these entities did not realize the strong temptation to remain in the bodies, having been merely brains for so long. Thalassa resents it, but is willing to do as her husband says. But Hennock has other plans.

First he hypnotizes Nurse Chapel to make Sargon/Kirk's body sick. He uses Nurse Chapel to inject Sargon's body with a formula different (with an intention to kill Sargon, as he knows Sargon will not allow him to retain the body) to what he is giving Thalassa and himself. He has no plans to leave Spock's body. Hennock finally poisons Kirk's body enough that he dies. But then Thalassa also decides that she likes Anne's body and decides to trade McCoy Kirk's body's life in exchange for his silence that Thalassa will never exit Anne. But she is stopped by Sargon, who had taken refuge inside the Ship. Thalassa understands the temptation of flesh and restores Kirk's body and returns Kirk into it. Similary Thalassa returns her body to Anne, and takes her consciousness inside the ship to join Sargon.

After wreaking havoc on the bridge, the crew discovers damage to the bubbles. Hennock has destroyed them. Spock is no more (since his consciousness was in one of those bubbles). Hennock, now in command of the bridge and mentally hurting and abusing everybody with his mind, laughs and turns to his 'slave' Nurse Chapel. "Inject Kirk with the poison" he laughs. But she turns and instead injects Hennock. He is aghast and open-mouth. "Noooo!" he yells. He quickly leaves the body but cannot find a host body, and dies. Then they hear a voice. It is Sargon, who restores Spock's body by getting rid of the poison. Spock is somehow alive but woozy. Kirk asks "What happened?" Sargon's voice explained. "There was no poison in the injection, Hennock simply thought there was, and fled, but his 'slave' was not under his power. We know now that the temptation of human bodies such as yours and Miss Mulhall's are too strong for us. Thalassa agrees but does not have sufficient power to respond as I do. She is with me here." Kirk is stunned.

But how did Spock's essence survive? Nurse Chapel has the answer. "His essence was inside me. For a while, we shared consciousness.". Kirk asks "Sargon, what will you do now?" Sargon says "We will perish, but you must tell the universe our story. we have only one request before we die." Kirk wonders what. "One moment for my wife and I to be human again, to say goodbye in flesh." Kirk looks at Anne. She agrees. Suddenly they are Sargon and Thalassa. They proclaim their love and engage in one final deep kiss. Then they vanish. There is Kirk in Anne's arms. He coughs. "Well, thank you, Miss Mulhall, for doing your duty." She smiles and says "My pleasure, Sir."