"Invasion of the City Builders"
After performing repairs to "rocket engine #4" that was damaged by a direct meteorite hit, the Enterprise continues on her research mission to the planet Alpha Z-21. Spock and Kirk meet to review the planetary history file. (Which is on reel-to-reel tape!) "One hundred years ago" Spock explains, "a deep galaxy ship managed to send these radio films back to a mother ship before apparently exploding! Evidently the spaceship was reconnoitering the planet at low altitude and used a telescopic camera!". The film shows a group of large mechanical devices. "A bank of ultra-modern machines" Kirk notes. "They appear to be working together in some sort of a mechanical complex" Spock adds, "Obviously the citizens are of a high civilization, being waited on by machines!".
Soon the Enterprise is in orbit around Alpha Z-21. Spock reports that the atmosphere is suitable for human life, and also detects large cities all over the planet. Like a glistening white comet, the Enterprise cuts through the atmosphere and glides across the planet. (within sight of the ground!) For hundreds of miles, the Enterprise cruises above a massive urban complex, without sign of life. Finally, the ship reaches the edge of the sprawling city. Kirk and Spock beam down to the surface, along with several security guards. "Our first operation should be to search for signs of life within those bizarre buildings!" Kirk tells the others. But even as the Captain gives instructions, a group of human-looking alien beings watches them from a nearby tower. "Friends or enemies" says the alien leader Krill, "they must await our attention! Plans can not be changed now!".
Within minutes, the ground starts rumbling, as if from an earthquake. A group of massive machines arrives in the area. As the landing party watches in awe, the machines set about creating building, roads, and extending the city. "But those machines" a security guard exclaims, "there's nobody operating them! It's positively eerie!". "Fantastic!" adds Kirk. "At this rate, Spock, it can construct a skyscraper in forty-eight hours!". The Vulcan sees the alien humanoids, and alerts the others. The group watches as the humanoids leap onto one of the machines, and try to stop it. The humanoids are thrown down by the machine, however. Kirk fires a "laser beam" at the machine, and short-circuits it. The huge vehicle withdraws, taking all the other machines with it. The landing party approaches the humanoids, and Kirk speaks to them in the interplanetary language, Esperanta. "We are friends" He tells them. "We come only to aid you!".
"Thank you, visitors from another world!" replies Krill, "we owe you a great debt!". Kirk asks Krill to tell him about the massive machines that just retreated. "150 years ago" Krill explains, "we were a proud and prosperous race of many millions! Today, a dozen of those mechanical demons rule the planet and threaten my village in the mountains yonder! It first began when our super-civilization began experimenting with machines to take over our everyday chores. As the years passed, our brilliant men of science created more and more ingenious machines. We were living in utopia! Our necessities were all provided for us! Business and commerce were systematized and carried out by our knowledge processing machines, far more astute than human minds! Our leaders smugly watched as our automated factory created the most complex machinery of all - The City Builder!".
The city-building machines return. "Three of them!" Krill exclaims. "There are only ten thousand acres of open land left. If they build on that, our planet is finished!". Krill's tribesmen come out of hiding, and he explains to them that the Enterprise men are friends who have come to help. "What do you think, Mr. Spock?" Kirk asks. "Can we do anything to stop them from continuing the city complex onward?". "Their vibrations register that they weigh over a thousand tons, Captain" replies the Vulcan. Kirk gets an idea. "If we made a trench with our earth disintegrators in their path we might mouse-trap them" he says, "with such weight they may become bogged down!". A 'soil destroyer' is brought down from the Enterprise. The device, which has a beam weapon at the end of a crane, is used to carve out a large section of land in front of the City Builders. Several of the giant machines fall into the pit, but are able to build ramps for themselves in order to escape.
As the machines emerge from the pit, Kirk and Spock turn massive force field generators on them. The machines are held for a moment, but then break free. "Fantastic!" Kirk says, awed. "The force field isn't powerful enough! They'll be coming to a big river soon! Surely that will stop them!". "No! No!" shouts Krill. "They have the concentrated intelligence of a thousand engineers within them! Every obstacle they face becomes a simple exercise in problem solving! You saw how they overcame your ditch! Watch when they reach the water's edge!". After the landing party crosses the river with Krill's people in small boats, they turn to watch the machines. The City Builders waste no time, and begin constructing a bridge across the water immediately.
The metal itself is too strong to cut, but Kirk aims his "laser gun" at a hoisting cable, in hopes that it will cause the girder it holds to fall near them. An angry Krill tries to stop them by attempting to ram their boat with his own. "You think you can take over, don't you?" Krill accuses them, "Win their respect with your false promises! Already they forget about me!". "A most illogical reaction" Spock comments as he leaps onto Krill's boat, knocking him down, "You need our help whether you want it or not!". Kirk's shot is successful, and the metal girder is recovered for analysis.
Mr. Spock quickly discovers a molecular flaw in the substance. "It breaks down almost instantly to a simple solution of NH2!" he tells the Captain. "Fantastic!" Kirk replies, "A metal that could probably withstand an atomic blast, yet a mild amino acid disintegrates it!".
Krill avoids red-hot rivets thrown in his boat by the huge machines, and fires a spray gun full of NH2 at the bridge supports. The structure sags, and then collapses, throwing three of the City Builders to the bottom of the river. "Huzzah!" Krill's people cheer as he returns, "You have saved us, Krill! We all owe you our lives!". Kirk gently reminds them that there are many more City Builders to be taken care of. "The Enterprise can locate the other City Builders for us" adds Spock. Shortly, the locations of all the other machines are established, and Krill and his people set out to destroy the remaining machines. "We'd like to go with you" Kirk says to Krill, "and see this job through, Krill, if it's alright with you!". "Captain Kirk" Krill replies, "I'd be proud to have you and Mr. Spock alongside of me!".
One by one, the City Builder machines are destroyed by Krill and his people. After the last one is gone, Kirk and Spock wish Krill farewell. "Well, I guess the hardest part is over!" Kirk says. "Yes, thanks to you and your men, Captain Kirk" replies Krill. "Now we can destroy the factories which produce the machines! And we still have to demolish so much of our useless cities and reclaim the land! It will take us generations! But we can and will do it! Good-bye Captain!". Kirk and Spock beam back up to the Enterprise. Later, they discuss the mission among themselves. "Starfleet Command is pleased with our report, Mr. Spock" Kirk tells the First Officer. "Naturally, Captain" Spock replies, "We preserved a civilization which would otherwise have perished! It is the only logical reaction!".