Before Planet of the Titans and Star Trek: Phase II, Paramount had rejected a Star Trek movie proposed by Gene Roddenberry called The God Thing and was looking around for a better idea.
John D.F. Black, who had written “The Naked Time” and served as story editor on Season 1 of The Original Series, pitched an idea in which alien worlds were using a black hole as garbage dump to the point where it threatened to destroy the universe. The Enterprise would be called in to prevent that from happening.
Harlan Ellison, who wrote “The City on the Edge of Forever”, pitched a story in which reptilians on the other side of the galaxy were changing time to destroy humanity. The Enterprise would travel back in time as well and Kirk would have to decide whether to wipe out an alien race in order to save his own.
Famed science-fiction author Robert Silverberg wrote a 51-page story treatment called The Billion Year Voyage in which Kirk and his crew are guided by a robot in their search of an ancient civilization known as the Great Ones.
Paramount was underwhelmed by all these ideas and decided to make a second Star Trek television series, Phase II — until it decided that its proposed pilot, “In Thy Image”, was the movie they had been looking for.
Having completed a mission on a world where all inhabitants are telepathically linked, Kirk, McCoy and Spock reflect on the human condition. The captain laments that humans are prisoners of their own skulls, unlike the Vulcans, who can mind meld. Spock is uncomfortable with this line of conversation and it takes McCoy to recognize this.
The purpose of the scene, Silverberg explained, was to establish Kirk as romantic, impulsive and sensitive; Spock as logical and keeping his deep-seated emotions in check; and McCoy as wise and perceptive.
It also hinted at what would become a central theme of the film: the spiritual isolation of non-telepathic species.
The Enterprise receives a distress call from Aurora V, where archeologists have been excavating a “Great Ones” site. The Great Ones where an ancient civilization with outposts thousands of lightyears apart. The archeologists report that they are threatened by invisible enemies. Spock, obsessed with the Great Ones, convinces Kirk they should investigate.
At the planet, Spock, Chekov and a Yeoman Baker beam down and meet with the archeologists, who report that things have been disappearing mysteriously, and that they have heard ghostly footsteps.
One of the archeologists, Kelley, is an empath and she reports she has detected cruelty, treachery and unscrupulousness. Spock suggests she may be talking about Klingons. The archeologist agrees. Perhaps they discovered a means to cloak themselves at another Great Ones site?
Spock, Chekov and Baker explore the site. Baker is abruptly grabbed by an invisible hand and carried away. Then the two men are attacked. Using his acute hearing, Spock is able to detect the location of their opponents and fight them off. Spock contacts the Enterprise to tell Kirk there are indeed invisible beings lurking there. He suggests sending down a search party with thermal sensors. Heat energy might reveal their opponents.
Kirk leads the away team and there is an instant attraction between him and Kelley. The team finds Baker and retires for the night. Days pass. Spock grows obsessed with the Great Ones culture. While investigating a mysterious globe, he is threatened with a pickaxe. Kirk fires his phaser and the invisible creature turns into a dead Klingon. Other Klingons attack, but the Enterprise crew prevail.
When everything has settled down, the group examines the globe found by Spock. When it is dropped, it reveals a holographic projection of the Great Ones: six-limbed humanoids with reptilian blood. We see pictures of their cities and their technology and of a Great Ones ship orbiting an astroid. Robots released from the ship carve out a vault in the astroid where one robot is left behind. Kirk and Spock reason that robot might still be there — and it musn’t fall into Klingon hands.
Spock is able to determine the location of the astroid. The archeologists beam aboard the Enterprise. Only the audience learns that some of the Klingons do as well.
The astroid is found orbiting a dying star. Kirk leads a landing party. They discover the vault, but the first person going inside — one of the archeologists — is killed in a blinding yellow light.
Kirk has the globe beamed down and turned on. That activates the robot in the vault, who takes the globe from Kirk and beckons the crew to come in. They are shown “a kind of travelogue of the Great Ones’ civilization.” The robot informs them it is waiting for the return of the Great Ones.
Kelley proposes to telepathically link Kirk with the robot to assure it of their good intentions. Kirk agrees, but the connection proves too powerful for him. Spock takes his place and succeeds: the robot agrees to share the location of the home world of the Great Ones.
He peers into space but is puzzled. The proper star isn’t there. He suggests they travel to a nearby system where the Great Ones had established a colony.
The crew discover live machines, but there is no organic life. The robot from the astroid contacts his counterparts on the planet, who immediately pull the Enterprise down to the surface. The robots are fascinated. Machines created by machines, they have never seen living beings before. It turns out the Great Ones perished some 84 million years ago, leaving only their machines behind. Their home world still exists but is now part of a Dyson sphere.
As the Enterprise makes its way to this location, strange things start to happen on board, but no one suspects the Klingons yet. When the ship arrives at the Dyson sphere, three Klingon battle cruisers attack. The Klingons stowed away on the Enterprise disable its weapons. It is only with the help of the robot that the crew survives.
The Enterprise enters the sphere and makes contact with the surviving Great Ones. Silverberg describes them as “hideously old,” their bodies “wrinkled and pouchy.” The crew find a variety of Great One artifacts, including a thought amplifier which permits communication between minds.
Back on the Enterprise, the battle resumes. The Klingons have not been defeated after all. Kirk uses the thought amplifier to find the Klingons hiding on his ship. He can also read the minds of McCoy, Spock and the others. It is an exhilarating experience.
Spock worries that the Great Ones’ technology is too advanced and argues that their home world should be declared off-limits. Kirk forwards this recommendation to Starfleet. Whatever they decide, Kirk has had a brief taste of the communal mind and feels he’ll never be the same again.