has been sent to explore a planet at the edge of the galaxy. Kirk, Spock, McCoy, and Sulu beam down and discover a number of buildings radiating power, but no inhabitants. As they go to investigate, Sulu finds a small mobile plant. When he picks it up, it stings him. Spock discovers that they are being scanned and they go in to investigate. They track the probe to a wall protected by a force field intended to protect that particular building. McCoy picks up an incredibly strong life humanoid life force, but before they can investigate, Sulu collapses in pain. McCoy determines he's been poisoned by the plant but can't identity the correct poison. With time running out, McCoy tries an antidote without success. However, four plant-like beings arrive and the leader, Agmar, offers his assistance. They have no choice but to accept and Sulu is quickly cured.
Agmar admits that they tracked the Enterprise
's arrival but have a fear of aliens. McCoy wonders how they knew their antidote would work on humanoid life forms and Agmar admits they've had contact with humanoids in the past. Kirk wonders where the rest of their people are and Agmar and the others take the landing party into a chamber containing larger versions of Agmar's people. Agmar explains that they are the generation before them, but a human came who brought sickness and death. He worked to save them from his disease. McCoy confirms that staphylococcus strains were the responsible for the death of Agmar's predecessors. Agmar himself was very young. Suddenly winged vegetative creatures fly in and attack. The landing party discovers their weapons are ineffective and Agmar informs them that there is a weapon deactivator in effect. The creatures grab Kirk, McCoy, and Sulu, and another one flies away with Spock. Agmar apologizes and informs them that the Master has searched many years for a specimen like Spock. He claims that all the worlds of the galaxy will share in peace and harmony. Kirk demands answers and a large human figure enters the chamber. He introduces himself as Dr. Stavos Keniclius 5 and tells them to return to their ship. The flying creatures return and Kirk has no choice but to return to the ship.
On board the ship, Sulu is unable to locate Spock or Keniclius while Uhura searches the database for any reference to Keniclius. She discovers some references in their history banks. The Phylosians' weapon deactivator proves effective against the ship's phasers as well. Kirk checks on Scotty to see if he's done with the special equipment he requested. McCoy arrives and says he has his grandfather's special gardening recipe ready. Uhura calls up the file on Stavos Keniclius, an Earth scientist from the Eugenics Wars 250 years ago. He was working on cloning to perfect the race and was banned for his theories, and then disappeared. Kirk figures that Keniclius cloned himself to continue the search.
The landing party prepares to go back to the planet. Scotty assures them the equipment is sound. Kirk orders Scotty to leave orbit to the Phylosians will think they left and lower their defenses. Kirk, McCoy, and Sulu beam down and discover that the remaining Phylosians are preparing a massive fleet. McCoy figures the disease wiped out the Phylosians before they launched their ships. They capture Agmar who says that Spock is an ideal specimen and cannot be returned. Spock will carry on their work: the Phylosians planned to depart on a great mission but their spore cells were destroyed. He offers to show them that Spock is safe. The landing party is suspicious but proceed. Agmar takes them into an underground chamber via a hover platform. As they proceed, they hear a flapping sound. Agmar cuts the lights and they hear the swooping creatures move in. They run toward the nearest light and find themselves in a chamber with Spock dying on a powered platform. Keniclius tells them that Spock will die, but a new Spock will live: a giant-sized Spock Two.
The swoopers enter the chamber and the landing party fires mechanical gas sprayers containing McCoy's weedkiller. The creatures collapse and Keniclius and Spock Two are forced to retreat. Kirk is unable to contact the ship and McCoy warns that Spock's mind is already gone, drained by the machine. Keniclius returns and explains that he cannot duplicate mental patterns, only physical cells. Kirk is unimpressed and tries to leave with Spock, but Spock Two blocks them. McCoy warns that Spock Two has yet to assimilate all the knowledge that his mind has been fed.
Aboard the Enterprise
, Scotty tries to contact Kirk and orders all power directed into a tight-beam transmission to penetrate the shielding. On the planet, Kirk challenges Spock Two to explain the logic to kill the original to create a duplicate. The Enterprise
communication beam penetrates the shield and Kirk gives the communicator to Spock Two. He eventually responds and Uhura gives him the additional information she's found. Keniclius' later writings indicate he planned to use his perfect race as a galactic peacekeeping force. Uhura cuts off just before the ship's power burns out.
Kirk tells Keniclius that the galaxy has always acquired a large degree of peace since the original Keniclius' era. The Phylosians arrive and McCoy wonders why they are helping Keniclius. Agmar informs them that they planned to launch a galaxy-wide peacekeeping operation and Kirk insists that they reached peace through mutual agreement. Keniclius refuses to save the original Spock. Kirk challenges Spock Two to determine if giant Vulcans could impose peace contrary to the Vulcan concept of IDIC. Spock Two agrees with Kirk but Keniclius refuses to hear any opposition. Agmar attacks and Sulu flips him using judo, while Keniclius destroys the machine holding Spock. Kirk goes to his aid but Spock Two intervenes and offers his help. He mind melds with the original, restoring his mind. Spock Two says that he will stay and assure that there is no army of Spocks. Keniclius wonders what he can do now and Spock suggests that Spock Two stay there with Keniclius and help the Phylosians bring their civilization back from the brink of destruction. Keniclius agrees and Kirk assures him he'll notify the Federation of his intentions.