“The Prisoner” was written by James Menzies as an episode for Star Trek: Phase II before the series was canceled and converted into The Motion Picture.
The plot — an alien masquerades as Albert Einstein and takes over the minds of the Enterprise crew — is reminiscent of “Return to Tomorrow” and “The Savage Curtain”. In the former, aliens similarly want to use the bodies of the Enterprise crew as hosts; the latter begins with the image of Abraham Lincoln appearing on the viewscreen.
The Enterprise‘s communication channels are infiltrated by a variety of voices which are completely unrecognizable at first. The bridge crew can make no sense of what the voices are saying until Xon identifies them as coming from Earth. Kirk believes he hears something familiar and asks Uhura to play it back separately. It is a speech by Winston Churchill. Then they recognize the voices of Gandhi and Harry Truman and Albert Einstein — who appears on the viewscreen when the voices stop. The scientist appeals to Kirk for help.
Einstein claims he and other Earth scientists have been kidnapped and kept alive by an alien force. Kirk is skeptical but orders the Enterprise to investigate. When they arrive at the coordinates, Einstein beams aboard together with Marie Curie, Robert Goddard, Karl Jansky, Buster Keaton and Max Planck.
Using his tricorder, Xon scans the group and announces that they are not really lifeforms at all but rather perfect illusions. Kirk orders the Enterprise to leave and the six figures disappear. But something is holding the ship back. It cannot break free.
Einstein’s image reappears on the viewscreen. He tells Kirk he feels responsible for the atomic era and wants to return to Earth to turn it into a paradise. When Kirk refuses, the scientist uses his powers to mentally assault the entire crew.
Kirk agrees to beam down to the planet and is welcomed by Einstein’s voice. The voice explains it took on the image of Einstein because he is such a recognizable personality from Earth’s past. The voice identifies itself as Logos and explains it has become obsessed with humanity since it started to receive transmissions from Earth. Its intention is to assume the identity of all human life, beginning with Kirk and the crew of the Enterprise.
Meanwhile, on the ship, Xon wonders why all the personalities they’ve seen are from the early 1900s.
Kirk tries to reason with Logos, arguing that humanity has broken free of slavery and conflict. Logos doesn’t believe him. Earth lives under constant threat of nuclear annihilation, he says. It turns out the last transmission he received was from 2024.
Kirk’s attempts to change Logos’ mind are in vain. He is thrust against an electromagnetic conductor and held in place as “programming” is initiated. According to Logos, millions of facts from the storage batteries will be fed into the captain’s brain.
A short while later, Kirk contacts the Enterprise and asks them to beam him back. He appears in the transporter room and is greeted by his officers. He explains that everything has been straightened out and orders Decker to set a course for Earth.
Decker is convinced that this is not Captain Kirk and states that he is taking command of the Enterprise. McCoy insists on examining the captain first. They proceed to Kirk’s quarters, where they too are programmed by Logos.
Surprised by the sudden course change to Earth, Xon contacts the captain to ask for an explanation. He is ordered by Kirk instead to gather the crew in the recreation room. When Xon looks for Decker and McCoy, he finds the two in a corridor walking back to Kirk’s cabin as though summoned telepathically.
Xon has Scotty turn down the temperature in Kirk’s room, which succeeds in rendering the three officers under Logos’ control unconscious. He then comes up with a plan. Since Logos lost contact with Earth more than two centuries ago, there is a lot of information he is unaware of. Xon has the computer feed all of that Earth history through the intercom. The amount of knowledge short-circuits Logos’ mind, which departs Kirk’s body and the Enterprise.