Executive Producer: Gene Roddenberry
Producers: Gene L. Coon + John Meredyth Lucas
Associate Producer: Robert H. Justman
Script Consultant: D. C. Fontana

Writer: David Gerrold
Dir.: Joseph Pevney

Captain Kirk: William Shatner
Science Officer, First Officer, Cmdr. Spock: Leonard Nimoy
Dr. Leonard "Bones" Mc Coy: DeForest Kelly
Lt. Sulu: George Takei
Lieutenant Commander Montgomery Scott: James Doohan
Lieutenant Uhura: Nichelle Nichols
Nurse Christine Chapel: Majel Barrett

Cyrano Jones: Stanley Adams
Captain Koloth: William Campbell
Arne Darvin: Charlie Brill
Nilz Barris: William Schallert
Mr. Lurry: Whit Bissell

Stardate: 4523.3

The Enterprise is to accompany a container ship loaded with quadrotriticale, a very special grain, the only kind growing on a certain planet. For this duty Kirk meets with the Undersecretary of Agricultural Affairs, Nilz Barris, and his assistant, Arne Darvin. All are aware of the presence of the Klingons, who have the same right to rest on Station K–7. Their ship with captain Koloth being at K –7 at the same time shows they, too, are interested in the grain. Barris is extremely worried about the Klingongs' presence. He even sent a distress call to have the Enterprise come to the station before the scheduled time and holds Kirk responsible for any damage they might cause. Kirk is quite annoyed by that; he knows that he can't prevent the Klingons to beam down and tries to win captain Koloth's collaboration to mantain order, but, as soon as the two crews meet on the station, it is trouble. Scott manages to stay cool while a Klingon officer calls captain Kirk a puppet and a Denebian slim devil, but when he calls the Enterprise a garbage can the engineer starts a brawl. Kirk is not pleased. 

On K-7, Cyrano Jones, a traveling dealer, is selling little, round furry animals (tribbles), which spend their time eating and multiplying. They are cute and lovely. Lt. Uhura is fascinated by them and takes one on board. The morning after she finds herself with about ten small furry creatures. Jones did not tell her that the more the tribbles eat the more they multiply. In no time the Enterprise is overpopulated by tribbles and easily Spock calculates the birth rate and shares the information about how many they will be in a day, a week ... 

On the station, things are even worse. The tribbles have invaded the quadritricale store and the Undersecretary is mad at Kirk for not being able to properly protect the precious grain. When Kirk goes to inspect the bin, he's overwhelmed by a cascade of little furry things. Spock discovers that the tribbles show antipathy against Klingons and react hostile, like spitting at them, when encountering one or several Klingons. He also finds out that most of the tribbles that had entered the bin where the grain is stored are now dead. Thanks to the tribbles' hostile reaction a Klingon spy is discovered: Arne Darvin; and thanks to their great attention for food it is discovered that the grain had been poisoned.  It's found that they have died of the poison in the grain. Arne Darvin was Nilz Barris' main collaborator, which puts the annoying Undersecretary in a quite embarrassing position. 

Cyrano Jones is condemned to collect all the tribbles still infesting the station and dispose of them. On the Enterprise, when the landing party beams up end enters the bridge, tribbles are nowhere to be seen, even though they kept on multiplying. Scotty had found a solution: He's beamed them on board of the Klingon ship.