"The Peril of Planet Quick Change"
The Enterprise is cruising through "Galaxy Telpha Z", when Mr. Spock discovers a strange planet that is changing its chemistry every five or ten minutes. Metallic matter is changing to foliage within an instant, and then back again! He also detects the presence of "tiantianium" on the planet. "Rocket power on full" Kirk orders. "Let's sail in close and give that palpitating planet a telescopic once-over!". As the ship passes near the planet they observe cascades of water bursting up suddenly from underground, and forests sprouting up from deserts! Kirk, Spock, McCoy and a security guard beam down to the surface to investigate. "Dangerous as it is" Kirk says, "the presence of that TI metal makes the risk worth taking!".
Spock tells the others that he has calculated a way to extract the tiantianium from the planet. He asks the Captain to have a list of special materials beamed down from the Enterprise and then begins working feverishly on blueprints for an unknown device, unwilling to discuss it with anyone. He directs construction crews from the Enterprise to begin building the device beside a nearby crater.
The alien light forms that swirled about the crew earlier have settled in Spock's head - communicating with him telepathically. "How?" Spock asks them, "When did this happen to your planet?". The thought waves of the six light forms fill Spock's brain, revealing their secrets to him. "It occurred many sun centuries ago!" the voices say. "A giant meteorite pierced our planet in its one weak area, where the structure was thin. It went through our crust, mantle, outer core and into the molten mass of our inner core! As the chemical reaction reached the surface these terrible changes began to occur, and our bodies began to vaporize". "We needed physical form to undo the damage done" the voices finish, "and your Vulcan body was the only one we could enter due to its chemical make-up!". "I understand" Spock says. "The evolutionary processes of your planet were speeded up trillions of times over! But, should we correct the problem, will you revert to your normal selves?". "Yes!" the voices reply.
The voices lead Spock to an underground cavern, where they had hidden special chemicals to neutralize the meteorite, before their people vaporized entirely. Spock takes the chemicals back to the construction site, and places them inside his device; a huge rocket, pointed down inside the crater. "Those chemicals" Kirk asks, "where did you get them, Spock?". "I had them in an emergency kit" Spock lies. "Come! Prepare to hoist the rocket out into the exact center of the volcanic crater before blasting it downward!". However, just as the rocket is moved into position, another series of earthquakes rock the planet, and endangers the construction team. "We've had it, Spock!" Kirk shouts. "Whatever your theory was of stilling this nightmarish planet is gone with the alien wind. If we get out of this with our necks, I'm ordering an immediate teleportation back to ship!".
The alien voices direct Spock to lead the crew to safety. Soon the tremors pass. "The upheavals have subsided!" Spock says. "We can continue 'Project Planet Still'!". "Now he's got a code name for the operation!" says Kirk to himself. When the rocket has been re-aligned, Spock fires it into the mouth of the crater. The rocket penetrates to the core of the planet, and a huge cloud of chemical gas gushes out. Alien forms begin rushing out of Mr. Spock's head and rematerializing! Soon, a group of men wearing animal skins and carrying swords, stands before the Enterprise crew. "Your brilliant Vulcan colleague was our host, Captain Kirk!" says the leader of the men. "Through his body we were able to save our world! And now we shall make repayment by giving you the vital ore you seek! My name is Zarman!".
Zarman and the other newly-reformed inhabitants of the planet lead Captain Kirk, Mr. Spock and the others to their city, Hidenna, which has also rematerialized nearby. Zarman shows them the wonders of the town. "All over our planet, one by one, other towns are rising!" he says. He then takes them to a large tiantianium deposit, so they can extract the precious ore to take with them. While the others begin collecting the ore, Mr. Spock wanders away, dazed. "My head, something is not correct" Spock thinks to himself. "My mind is still not clear, concise in thought. Wait! Six light forms entered my body. Yes. Yes, six entered, but only five left!". "That is true Mr. Spock!" booms another voice inside the Vulcan's head. "I am still within your body, your mind. Yes, within your very soul, Mr. Spock!"
Spock returns to the others, who have finished the task of mining the ore. He tries to warn Kirk about the being that still inhabits his body, however Textra overwhelms his mind and prevents him from speaking. The Vulcan's body is racked by spasms as he fights for control of his mind with the alien. "Spock's in shock" Kirk says to the others. "The reaction of the alien light creatures within him, no doubt!". Zarman apologizes to Kirk again for entering Spock's body, not realizing that one of his people still inhabits the Vulcan's mind even as he speaks. Kirk wishes Zarman well, and the landing party beams back up the Enterprise with the Tiantianium ore.
Back on the ship, Spock's behavior turns from unusual to bizarre, as he and Textra fight for control of his body and mind. He attempts to write a note to the Captain, warning him of the danger, but Textra is able to overpower him and prevent him from doing so. Worried, Kirk and McCoy drag Spock to sick bay, and confine him in a bed with restraints. The Doctor gives Spock a sedative, and the Vulcan lies alone in sick bay - his body sleeping, and his mind fighting against Textra! Spock is able to get control of his body enough to make his way to the "teleportation chamber", deliberately avoiding thinking about his plan so that Textra cannot read his thoughts.
"So that Textra was planning to use you as a host forever" Kirk asks Spock later. "Yes Captain" Spock replies. "I'm glad I thought of neutralizing the teleportation machine! It made my body so unstable there was nothing for Textra to hang onto!". "I always knew there was something unstable about you Vulcans" quips McCoy. "Dr. McCoy" Spock answers, "it seems to me you Earthmen can achieve instability even without a machine!".