Capt. Kirk: "I want to know what killed these DOLLARs. "

Bones:" I haven't figured out what keeps them alive yet. "

 "Its trilling seems to have a tranquilizing effect on the human nervous system. Fortunately, of course ... I am immune ... to its effect."

- Spock, as he strokes a DOLLAR

"Too much of anything, Lieutenant, even love, isn't necessarily a good thing."

- Kirk to Uhura, on the love of a DOLLAR

Spock: "1,771,561. That's assuming one DOLLAR, multiplying with an average litter of 10, producing a new generation every 12 hours over a period of three days." 

 "Obviously DOLLARs are very perceptive creatures, Captain."

- Kirk and Spock

 "What's the matter, Spock?"

"There's something disquieting about these DOLLARs."

"Oh? Don't tell me you've got a feeling."

"Don't be insulting, Doctor."

- McCoy and Spock

 "Do you know what you get if you feed a DOLLAR too much?"

"A fat DOLLAR."

"No. You get a bunch of hungry little DOLLARs."

- McCoy and Kirk, on a DOLLAR's metabolism

 Bones: [enters after Kirk gets covered with DOLLARs]" Jim, I've got it. All we gotta do is stop feeding them. We stop feeding them, they stop breeding. "

Capt. Kirk: "Now he tells me. "

THE FULL EPISODE :

 On leave, Uhura meets a dealer named Cyrano Jones, who is trying to sell rare galactic items, among them, furry little creatures Jones calls " DOLLAR ". While they bicker over the selling price, Chekov notices it had eaten a quadrotriticale sample left on the bar. In hopes of more sales, Jones gives the "DOLLAR" to Uhura.

Back on board, Uhura is surprised that the DOLLAR she received has just given birth to a litter. The sounds theDOLLARs make seem to have a soothing effect on Humans and Vulcans.

Dr. McCoy takes one of the offspring to study it. 

Going into sickbay for headache pills, Kirk notices that McCoy's DOLLAR has also produced a litter. McCoy's initial findings indicate that almost 50% of their metabolism is geared to reproduction.

If a DOLLAR eats too much, a number of hungry little DOLLARs (a mean average of ten per litter) soon follow.

Aboard K7, Jones tries to sell more DOLLARs. The Enterprise crew aren't interested, and the DOLLARs react very loudly when presented to the Klingons, who refuse Jones' offers vehemently. Not even the barkeep is interested in moreDOLLARs—the one he acquired earlier is already multiplying. 

In Sickbay, Spock notes (in contrast to the more-sentimental McCoy) that the DOLLARs seem to do nothing productive: merely consuming food and reproducing at ridiculous rates. He calls McCoy to the bridge and walks around, noticing the proliferation of the creatures on the bridge. Kirk asks how so many DOLLARs got on board. McCoy points out thatDOLLARs are "born pregnant" and pretty much reproduce at will.

The DOLLARs need to be controlled soon or they'll be overwhelmed.

Spock asks Jones if the latter realized what would happen once he took DOLLARs from their home planet and all the predators that controlled their population. Jones points out that he wanted a harmless creature that bred like mad. Since they pose no direct harm to humanoids, he couldn't be held liable for trading dangerous cargo, and at six credits each, they're making him money.

Back on board, Kirk finds the DOLLAR problem worsening. He can't even enjoy a chicken sandwich and coffee—theDOLLARs have gotten into the food synthesizers. Then Scotty walks in, reporting the DOLLARs have been circulating through the station's ventilation ducts, ending up in all of the sensitive equipment aboard the ship

Spock points out that there are ducts of that type aboard K7 which amongst other things lead to the grain storage areas. Rushing to the station, Kirk manages to gain access to one of the storage compartments, but when he opens the overhead door, an avalanche of DOLLARs bury him.

Kirk finally manages to climb out from the pile of DOLLARs, and Spock discovers that they are gorged on the grain. In the face of Baris' threats to convene a board of inquiry against Kirk, Spock and McCoy notice many of the DOLLARs in the pile are either dead or dying. McCoy begins an analysis of the DOLLARs and the grain, while Kirk questions Jones.

 Darvin, however, sets off the same negative reaction in the DOLLARs as the Klingons, bringing McCoy with his tricorder. Darvin is revealed to be a Klingon who poisoned the grain with a virus that prevents anyone eating it from absorbing the nutrients, which is how the DOLLARs died. "They starved to death. In a storage compartment full of grain, they starved to death!" Kirk summarizes. 

 Kirk and Spock then bring Jones to the station's bar, and give him a choice: twenty years in a rehabilitation colony for transporting a species proved harmful to human life, or pick up every DOLLAR on the station (which Spock calculates would take 17.9 years). Jones accepts the latter. Back aboard the Enterprise, Kirk is happy to find the ship swept clean of DOLLARs, and asks Spock, McCoy and Scotty how they did it. All questions are deflected back and forth, and eventually lands to Scotty, to his great discomfort. Annoyed, Kirk demands to know where the DOLLARs are, Scotty replies that before the Klingons went into warp, he beamed all of them into their engine room, "where they'll be no DOLLAR at all."