Invasion of the City Builders
AUTOMATED DESTROYERS! Minutes to save a dying planet!

Summary 
"Captain's log, stardate 20:14.6 – Mission has been delayed by direct meteorite hit on rocket engine #4 – Repairs being made – Expect to continue toward objective shortly…"

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 beams 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 NH2, 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!"
 - Kirk and Spock, watching the construction

"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!"
 - Kirk and Spock, talking about Krill

Background information 
 This story was reprinted in Enterprise Log 1 and a later reprint of that volume from the Star Trek: The Key Collection series.
 The small inset photo on the cover is from .

Creators 
 Based on Star Trek created by Gene Roddenberry
 Writer: Dick Wood
 Artist: Alberto Giolitti, Giovanni Ticci (inks)

Characters

Regular and recurring 
 James T. Kirk  Enterprise captain.
 Spock  Vulcan Enterprise exec and science officer.
 Leonard McCoy  Enterprise chief medical officer. He did not appear in the story, but was pictured on the cover.
 Uhura Enterprise communications officer. Uhura did not appear in the story, but was pictured on the cover.
 Janice Rand Enterprise yeoman. Rand did not appear in the story, but was pictured on the cover.

Others 
 Krill  Leader of the alien race on the planet Alpha Z-21.
 Zika  Another of the aliens.

References 
 City Builders  The machines that were designed to build cities in a short period of time.
 Alpha Z-21  A planet also known as "Planet Questionmark" where the Enterprise is heading for research. Its proper name is "Zarta".
 Esperanta  The interplanetary language.

fr:Invasion of the City Builders (Gold Key Comics)
Comics