Invasion of the City Builders " AUTOMATED DESTROYERS ! Minutes to save a dying planet ! " Summary . The is heading toward " Planet Questionmark " to do research based on radio films that had been sent out a hundred years before from a deep space starship that exploded shortly afterward . The films show a series of ultra - modern machines working together in a mechanical city . Two lunar hours later , the " Enterprise " arrives at the mystery planet . The vessel drops into the atmosphere , its rockets firing as it passes over endless miles of an urban complex , without a sign of life . Eventually , the ship finds an open area ahead where they can land . The ship moves back up into orbit , and a landing party beam s down to the surface . On the planet , they notice that the super - city ends abruptly , as if it was cut off with a knife . Inside one of the buildings , aliens have noticed the party , and asked their leader , Krill , what to do . Just as he began to answer , the ground starts shaking and enormous machines come into view , but they are completely empty . The machines are building roads and buildings onto the city , extending it into the open area where the " Enterprise " crew is . They spot the aliens , leaping onto the machines , but just as they do , the machine shakes them off . Spock recommends using their laser rifles on the machines , and Captain Kirk fires at one , causing it to short - circuit . After this is done , they approach the aliens and the leader introduces himself as Krill , leader of the civilization on Zarta . He tells them that 150 years before , they were prosperous and plentiful , but they began using machines for their everyday chores . Over time , the machines became bigger and more ingenious , and then everything was automated , until one day , the machines began to design themselves , ending with the city builders . The aliens could do nothing to stop the expansion of cities and machines . Things were all right until a small gear in an agricultural machine broke , and the food could no longer be harvested , processed , or distributed , and the aliens had forgotten how to fix it . Shortly after the story was finished , the machines returned . Spock and Kirk bring the soil destroyers into play , attempting to trap the thousand - ton machines in pits and bogging them down under their own weight . However , the plan fails as the machines begin to dig themselves out of the holes . Kirk suggests using a force field , and they entrap the machines , but the machines continue to build . The force field isn ' t strong enough to stop them however . Krill explains that every obstacle the machines encounter simply becomes a problem to solve , just as the river that they soon encounter . Spock calls up to the " Enterprise " for a metallic test kit to be teleported down . Spock plans to analyze the metal because logic tells him that " " every substance can be broken down by some process . " " On a boat , Kirk tries to use his laser rifle to cut a cable so that they can get a sample of the metal . Krill drives boat into the side of them , accusing them of trying to take control of his people . Spock leaps onto Krill ' s boat , distracting him , and allowing Kirk to fire at the cable . When they get the girder , Spock tests it desperately , but everything fails , until he pours a solution of NH , a mild amino acid , onto it , which dissolves the metal almost immediately . Vats of the chemical are beamed down , and Kirk asks Krill if he will help them attack the machines with the acid . However , Krill insists on doing things alone , because he has held his people together alone for years . As Krill fires the acid at one of the towers of the bridge , the metal begins to melt . However , one of the machines detects something wrong and begins firing red - hot rivets at Krill ' s boat . He drives his boat under the bridge , and the bridge collapses onto his boat . However , Krill manages to survive , and his people fish him out of the water . With the help of the " Enterprise " , they pinpoint all of the other machines and attack them , destroying all of them . It will be many years before Krill and his people can reclaim all of the land by destroying the city , but they are willing to do the work . Memorable quotes . " " Gr - Great novas ! The power of that force field has stopped alien armies on other planets . . . but that monster city builder i - is breaking through it ! " " " " And " still " building structures as it does so , captain . Remember , they are made of a metal unknown to us ! " " " " Why that egomaniac ! He ' s more concerned with keeping his power than saving his people ! " " " " He ' s very much like you Earth people in that respect , Captain ! " "