The Enterprise approaches the planet Floe I on a routine population survey assignment. Sulu voices his hope that the Floe natives don't make the survey difficult. Scotty agrees. "It's hard enough to imagine being down there, our bones shivering away in the coldest planet of the Federation!". Kirk, Spock, and Dr. Krisp (a lovely young woman with short brown hair) put on special "anti-freeze" uniforms in preparation to go to the surface. Dr. Krisp reminds Kirk and Spock that the natives of the planet can communicate with telepathy. Kirk comments that Federation annals indicate nothing unusual on the planet, so the survey shouldn't run into any trouble. Privately, Spock thinks it is unusual that Starfleet would waste the Enterprise's time on something so routine as a population survey if there weren't anything unusual about the planet Floe I. He wonders what it might be.
The landing party makes it's way to the bridge, wearing the special suits. Sulu tells the Captain that the people on the planet have radioed back and are expecting them. Spock hands out phasers to the group. Dr. Krisp also brings along some "tele-radios". "Maybe Scotty can radio down some background music and make the job less boring" she says. Soon, the landing party beams down to the surface. When they arrive, they see huge igloo-like purple structures dotting the landscape.
Her shell removed, Amara asks the Enterprise crew to follow her. Spock begins to wonder how many of the population actually did survive the climate changes and adapt - and how many perished. He now understands why a population survey makes sense, and suspects that the Federation has not been entirely fooled by the "turtle-people". As soon as Spock has these thoughts, the turtle-people turn on the landing party. They are telepathic, and have read Spock's mind. "I'm afraid your mission won't be as pleasant as we expected!" Sunaro says. "Your learned colleagues have spoiled the fun!" Amara uses a flame weapon to melt the ice beneath their feet, and they are quickly taken captive.
On the Enterprise, Scotty and Uhura (who is mistakenly depicted as a white woman) are worried about the landing party, but agree to wait a little longer before contacting them. At that moment, Kirk calls the ship. He is being held at gunpoint by the turtle-people! They force him to ask the Enterprise to send down Lieutenant Uhura to "lend a hand" as soon as possible.
Kirk asks Sunaro why he has taken them captive, and tells him that they have come in peace from the Federation. "Our only peace comes in survival! Survival from day to day!" says Sunaro. He tells Kirk how they once used to be a very happy people. Their grass was green, their civilization was admired by other worlds, and their bodies were perfect. "And then," he tells Kirk, "came a fate so unexpected... a curse! I believe Mr. Spock and Dr. Krisp are catching a view of it right now!".
Deep in the city, Spock and Dr. Krisp are imprisoned in a transparent cage that hangs above a large number of blue turtle-people being kept in a fire pit. The pair are dumped into the fire along with the others, however their environment suits protect them from harm. One of the blue humanoids explains that they are being tortured because they evolved with a different skin color than the others of their race. They have been denied the "mutation speeding serum" which the other inhabitants of Floe take. This serum prevents their blood from slowly freezing them to death! Only the fire keeps them alive. Spock discovers a large video screen that is capturing the torture. He realizes that the humanoids are being killed slowly as an example to others who do not follow the orders of Sunaro and Amara! Spock offers to help them escape if they will show him where the mutation speeder serum is stored, and they agree.
The Enterprise crew travels to the serum production chamber and overpowers the guards there. They tell the slaves to take the serum to their people who are being tortured. Seizing the opportunity, the blue slaves begin attacking their captors all over the city. "Captain, I'm afraid we've been the instruments of a civil war!" Spock says. "The Federation would find this most displeasing". Kirk quickly orders the Enterprise to beam the landing party back on board.
Scotty asks Kirk why he left the people of the planet in civil war. Kirk explains that although it is against Federation law to interfere, the planet Floe is doomed to end in ice and then nothingness as it races away from it's only source of energy - it's sun. Even the inhabitants themselves knew that they had but a few days to live. Scotty questions why the people would have a war with each other if they knew they were going to die anyway. "I believe Earth people have a word for it, Scotty" says Spock. "A word missing in the Vulcan dictionary. That word is HOPE!".