More Tribbles, More Troubles (1973)

While the USS Enterprise escorts two robot cargo ships carrying quintotriticale, a new seed grain, to famine stricken Sherman's Planet, it encounters a Klingon battlecruiser pursuing a Federation scout ship. When the Enterprise rescues the pilot, the Klingons attack with a new energy weapon and demand that the pilot be handed over to them.

The pilot turns out to be Cyrano Jones, an intergalactic trader well known to Kirk and crew from "The Trouble With Tribbles". The Klingons desperately want Jones for introducing the tribble to Klingon planets where it has become a major pest, and for stealing a glommer, an animal the Klingons were breeding to prey on the tribble.

Kirk expresses sympathy for the Klingons' plight but refuses to hand over Jones, a Federation citizen.

Jones is now selling tribbles genetically engineered to omit their ordinarily unrestrained multiplicative proclivities. Dr. McCoy discovers that although Jones' "new" tribbles don't reproduce, they still have ravenous appetites. Instead of reproducing, they now grow hugely fat.

Kirk and Spock manage to counter the Klingon weapon. Now at a disadvantage, the Klingons offer to settle for just the glommer. Kirk agrees and returns it—along with several well-fed tribbles that scare it away. McCoy injects the remaining tribbles on the Enterprise with a serum to slow down their metabolic rate. They are finally "safe".