The Enterprise arrives at the "Trading Post," a small settlement on a sparsely-populated planet, a jumping off point to outlying frontier worlds. We soon learn that the Barth Neo-Corn Corporation has a monopoly on farm lands on the planet Barth. A rival company, headed by Damon Jones, hopes to use their warehouse full of quick-growing mutated wheat grain at the Trading Post as a seed crop to compete with Barth Neo-Corn.

Jones and his twitchy assistant, afraid that their grain may be sabotaged, ask Kirk to guard the warehouse until their freighters can pick it up three weeks hence for transport to Barth. Kirk agrees to do so. The planet is lovely and pastoral and his crew needs shore leave. Additionally, a three-week layover will give Scotty time to make many small repairs that the Enterprise needs.

The Trading Post is a small town, comparable to an American frontier town. Soon Enterprise personnel are browsing at the many shops in town. Kirk and Spock stay near the town transporter center (the planet does not have a spaceport), keeping an eye on the arrivals and departures.

A small scout ship enters orbit and Cyrano Smith, a colorful "locator" (one who locates uninhabited planets and sells the rights to them to the large corporations), beams down. He is a roguish sort, and Kirk issues a warning that he be watched at all times. Smith is offended, but what can he do.

Kirk and Spock watch as Smith enters a shop and attempts to sell a "Fuzzy" to the owner. It is small, furry, greenish-gold, with no legs, no eyes, only a small cute mouth. This one is about the size of a tennis ball though Smith says he has seen them as large as volleyballs. The shopkeeper agrees and buys the fuzzy, tells him he wants more to sell. Smith says he must beam up to his ship to bring more down.

Kirk and Spock follow Smith and discover that he actually has several more fuzzies in his pouch. He goes to several shops, sells each owner a fuzzy, and soon has a total order of five hundred.

Kirk and Spock, back on the Enterprise, discuss Smith. Kirk says he’s just a harmless con man. Spock is not so sure...

Kirk checks in with Damon Jones and his twitchy aide. The grain is still fine, nothing has happened to it.

Smith delivers the five hundred fuzzies, and the shop owners sell out almost immediately, demand more, which amazingly Smith is able to instantly supply. Janice Rand even buys a fuzzy and brings it on the Enterprise. Soon the whole crew has bought one. The fuzzies eat everything in sight. Then Janice’s fuzzy has a litter of ten, and Scotty complains that he saw one in the machinery.

McCoy dissects one, discovers they are asexual, almost born pregnant. Kirk confronts Cyrano Smith. He knows that Smith knew about their propensity to give birth when fed too much. Smith hems and haws and leaves the trading post.

Kirk returns to the Enterprise and...Oh Boy! Rand’s fuzzy’s litter has had a litter, and everybody’s fuzzies are giving birth like crazy--they’re everywhere--on tables, under tables, on chairs, under chairs, on beds, under beds, in corners, in the engines, in the galley, in Kirk’s coffee...

The ship’s angry cook takes Kirk and Spock to his flour bins--the fuzzies have completely devoured the flour. Nothing in the bins but fat fuzzies.

Kirk and Spock stare at the empty bins, both think the same thing at the same time... "The warehouse of grain!"

Kirk pries the granary doors open. Fuzzies roll out. The worst has happened. They have devoured the grain. Spock quotes the number of fuzzies exactly, and Kirk issues an order. "First, close that door! Second, capture Cyrano Smith!"

The Enterprise pursues Smith’s small vessel, beam him aboard and take his ship in tow. He protests his innocence, but Kirk is sure that it was he who destroyed the grain by putting fuzzies in the warehouse. He tells Smith to take his pleas of innocence to the Interstellar Commerce Commission.

When the Enterprise returns to the Trading Post planet, all are in for a shock. Every fuzzy in the warehouse is dead. McCoy determines that the grain has been poisoned. Had it been shipped to Barth, the crops raised from it would have killed untold millions.

The grain must have been poisoned before the fuzzies got in. Cyrano Smith is off the hook, and he fingers Damon Jones’ twitchy assistant as the real villain behind the poisoned grain.

The fuzzies are removed from the Enterprise, and Kirk orders Smith to stay at the Trading Post until he has removed every fuzzy there.

Back on the Enterprise, Kirk is relieved, says that he never again wants to see a fuzzy.

Janice Rand enters the bridge with two small furry balls. What are those things? Kirk yells.

"Earrings," Janice says as she clips them to her ears.

Everyone laughs. . . except Spock.