"To Err is Vulcan"
On Stardate 9126.1, the Enterprise is on a protocol visit to the planet Njura. The civilization on Njura had been colonial master of four other plants for nearly 1000 years until a generation ago a widespread rebellion grew which eventually touched off a bloody war. Finally, Njura's emperor Kring ended the conflict by signing a peace treaty. A democracy was set up with the former Emperor being elected president. The Enterprise entered orbit and Kirk, Spock and McCoy beam down to Njura which is celebrating its 10th anniversary of disarmament.
On the planet surface, the Enterprise men are escorted to meet the president who is tending his bees - a special breed that produces a superlatively delectable form of honey called varta.
After some refreshments Kirk, Spock and McCoy are given a tour of the city by a Dr. Mlany H. Rdal - a beautiful Njuran woman who is all at once an expert on advanced physics, electro-bionics and peaceful uses of subatomic energy.
The three men are impressed, and Dr. Rdal is impressed by the representatives of Starfleet, especially Spock. As they leave she says, "This entire tour should be of interest to you, Mr. Spock! You possess the most protean intellect!"
The tour the capital city takes them to a giant generator plant. "Eleven of these mammoth nuclear 'ovens' develop enough electricity for our entire world!" she tells them. The plants are also marvels of safety as there are 72 fail-safe devices built-in. In the control center of the power generating complex, Spock notes the gauges and after doing some mental calculations proclaims, "Yes! I'm certain of it now! Please, Dr. Rdal, you must close down generators five and six! They are approaching a critical condition!" Their brainy female host does not believe it possible, and so after failing to convince anyone that the "nuke-gens" will obliterate half of the planet if not shut down, Spock flings into action and shuts them down via massive breakers. Rdal is livid as Spock's actions have cut power to 100 million people.
Part II: the Plot Against Peace.
Spock puzzles over a huge planetary weather map and after several moments announces, "Dr. Rdal! At the risk of being thought a total alarmist, I must warn you that this city is in the direct path of a tornado!" Dr. Rdal is aghast, "Now, see here, Mr. Spock ..." and McCoy chimes in, "Oh, brother, not again!" After a quick call to the area in question, word arrived that no tornado conditions exist at all. Spock, wishing to prove his veracity, ushers them back to the wall chart room. There he and Dr. Rdal discover that apparently Mr. Spock misread the direction of an arrowhead showing pressure fronts. This second small error caused Spock to become even more puzzled. Their scientific guide is not amused and she says so. Kirk has had enough and orders Spock to beam back to the Enterprise to be examined by McCoy in sickbay later. Dr. Rdal is perturbed because her actions in some measure lead to Spock being humiliated and she bows out of the rest of the day's activities. Later, back in their hotel, Kirk tells McCoy that he had to reprimand Spock publicly to allow Dr. Rdal to save face since she seemed to do have cried wolf twice in front of her colleagues.
The next morning, the lovely scientist Dr. Rdal calls on Kirk and McCoy, and they join her in the reviewing stand overlooking the Peace Anniversary Parade. A great fly by of several large planes kicks off the parade. They aren't warplanes, but are crop spraying vehicles that playfully spray the crowds with a light perfume. A pyrotechnical display is next, in which a huge symbol of the Njura bird of peace is formed by red fireworks. Then, farmers march out, alongside huge farming combines which can do the work of hundreds of men. Dr. Rdal comments that not long ago, they would have been tanks if they hadn't found peace.
Oddly, the vast farm machines begin to heat up and glow - causing the operators to jump out of them and flee in front of thousands of awestruck onlookers. Then there outer skins melt away revealing that they were in actuality giant battle tanks. It seems that the parade of peace was a sham. McCoy spells out, "They've been secretly rearming against their old colonies ... but who exposed the plot, Jim?"
Then they noticed that down by a distant entrance, the police have detained - Spock! Rushing to the scene, Kirk, McCoy and Dr. Rdal arrived just as the president is saying to the police officer holding Spock, "Hold him for summary judgment! He'll learn how we deal with such treachery!" Kirk calls out the president, proclaiming that it was the president's plot that was exposed. The elder statesman unperturbed by the turn of events defiantly announces that he'll just move up the timetable and, "The reconquest shall begin a once!" Spock replies that he has just come from sabotaging the power generators and weather center - so without power and weather data there can be no invasion. After Kirk has determined that the president has recanted his quest for power, the starship captain suggests that a cover-up be staged blaming some unknown general turned traitor named Gmar. So with the aid of the ever-present television news cameras, Kirk announces to the cheering throng that a military plot has been smashed by their noble leader - president Kring!
As the events of the day come to a close, the starmen and their local guide, Dr. Rdal, relax over drinks. It turns out Kring's advisers set up Spock because his reputation indicated that he would have too soon have uncovered the facade of the peace parade. Kirk had actually sent Spock back to the ship to allow him to work freely to investigate the state of affairs on Njura.
McCoy had been kept in the dark because Kirk had feared there might be listening devices in the hotel suite. After discovering the camouflaged tanks, Spock planted thermal charges to melt and their outer coverings.
Back aboard the Enterprise, Spock laments about his seeming to have made two very public mistakes, "For a Vulcan, Dr. McCoy, that is two too many.!!"