is escorting two robot grain ships to Sherman's Planet to help the colonists survive. They break off to investigate a Klingon cruiser attacking a scout ship. The Klingons refuse to identify themselves and Kirk has Scotty lock onto the pilot with the transporter. The Klingons destroy the scout ship and Scotty tries to recalibrate the transporter, and then the Klingons turn on the Enterprise
. They unleash a new weapon, a field effect that immobilizes all power systems on the ship, including the transporters. However, the energy drain on the Klingon ship is also extreme, preventing them from firing. The Klingon captain, Koloth, finally contacts them. He demands that they turn over the pilot who is wanted for "ecological sabotage." They lose contact with the robot ships, which are out of range of the field effect. Kirk has Sulu use the freighters to ram the Klingon ship then gives Koloth a chance to release the Enterprise
. Koloth realizes what's happening and tries to split the field effect three ways, but only succeeds in overloading his generators. The Klingons disable one robot ship and then flee.
With systems restored, Scotty is able to materialize the pilot. Kirk, Spock, and McCoy arrive and recognize the pilot: Cyrano Jones, space merchant. And he has tribbles with him. They seal off the area and Jones claims the tribbles are safe. He explains that they have been bred not to reproduce. Jones explains that he managed to get a parole from Space Station K-7 where he was ordered to clean up all the tribbles, by obtaining a tribble predator known as a glommer. He demonstrates how it simply and cleanly eats tribbles, and claims he has no idea why the Klingons were chasing him. Jones denies being an ecological saboteur, but finally admits that he inadvertently sold tribbles to the Klingons. Kirk puts him into custody for various criminal violations.
McCoy examines a tribble and confirms Jones' claim: the tribbles simply get fat. Kirk is more worried about the new Klingon weapon, but Spock notes it immobilizes the Klingon ship that employs it. He concludes that the Klingons will return after recharging the stasis field, then disable the remaining robot ship. Scotty reports that they've transferred all the grain from the disabled ship to the Enterprise
cargo holds but the extra mass is taking up space and power. The Enterprise
continues for Sherman's Planet, while the tribbles roam the halls and the glommer tries to eat them.
The Klingon cruiser approaches and immobilizes the Enterprise
, while Kirk brushes a larger tribble out of his command chair. Kirk uses the robot ship as a decoy but the Klingon cruiser disables the grain ship and leaves the cargo intact. It then opens fire on the ship, and the shock breaks open the containers of grain. The tribbles eat the grain and grow larger, while the cruiser fires a few shows and then leaves without using the stasis field. Spock suspects the attack served some other purpose. Kirk puts a tractor beam on the grain ship but Spock notes that taking it under tow will use up a great deal of their power. He suggests throwing tribbles at the Klingons.
As the glommer struggles to eat the growing tribbles, Scotty reports that the grain is loose in the hallways. Kirk calls Jones to the bridge and notes the tribbles are growing. He begins to wonder if that's the ecological sabotage Koloth was talking about. The Klingon cruiser comes around for another attack and McCoy arrives on the bridge but Kirk puts him off. They release the robot ship from the tractor beam and fire photon torpedoes, but the Klingons unleash the stasis field, negating the torpedo and immobilizing the Enterprise
. Koloth angrily demands they turn Jones over and Kirk refuses. Koloth implements Boarding Plan C, while Kirk implements Emergency Defense Plan B, which Scotty has never heard of. The Klingon marines go to their transporter room... and discover it's filled with giant tribbles. Kirk contacts Koloth, who doesn't know about the tribbles. Kirk informs him he's immobilized their ship, and a giant tribble arrives on Koloth's bridge. He reluctantly explains that Jones stole the glommer, a prototype Klingon genetic construct, that they need back to get rid of the tribbles Jones sold them. Koloth has no need of Jones and Kirk agrees to beam the glommer over to Koloth. Jones is upset, until Scotty offers to beam him over with the glommer. Jones hastily withdraws his claim.
Koloth receives the glommer and release the Enterprise
, Kirk is satisfied with the relative ineffectiveness of the stasis field and Spock notes tribbles are more effective. McCoy reports that Jones bred the tribbles not to reproduce, but the giant tribbles are actually colony creatures. Koloth takes the glommer to his engineering room but the glommer runs in panic from the giant-sized tribbles. Koloth tells his lieutenant to blast the tribble and he does. It splits apart into hundreds of tribbles... and they start growing.
Aboard the Enterprise
, McCoy comes up with an injection that will break apart the colony tribbles and keep them from breeding. Kirk notices one giant tribble in a Jefferies tube and McCoy says he injected it. It breaks apart, burying Kirk in tribbles... again.