A small, fast-breeding creature invades the Enterprise and a space station in the middle of a grain dispute.
On Deep Space Station K-7, a trader sells a small furry creature, the tribble, which purrs soothingly and breeds at an incredible rate. As the animals overrun the Enterprise, Kirk deals with a dispute over a grain shipment, the Klingons and an irritating Federation official.
One of the funniest episodes, it was written by David Gerrold and has become a classic. The tribbles' reaction to Klingons gives away an enemy spy, and the barroom brawl is a highlight. Fans remember the avalanche of tribbles falling on Kirk, and the creatures have appeared in later Trek stories.