6. Trials and Tribble-ations

Agents from the Department of Temporal Investigations (DTI) arrive at Deep Space Nine to question Sisko about an incident involving the Defiant. The ship was sent to Cardassia to retrieve the sacred Bajoran Orb of Time, stolen during the occupation. On their way back to Bajor the ship takes on a passenger, a human trader named Barry Waddle. Suddenly, the ship finds itself more than one hundred years in the past and approximately 200 light years away from its previous location, near Deep Space Station K7 and the USS Enterprise. They discover that the hitchhiker is actually Arne Darvin, a former Klingon agent disguised as a human who was caught by Captain Kirk on K7 after poisoning a shipment of grain.

Fearing that Darvin used the Orb of Time to alter the past to prevent his capture, the crew dress in period uniforms and investigate the Enterprise and K7. The ship and station are infested with tribbles, small furry creatures that reproduce rapidly. They attempt to interact with history as little as possible while searching for Darvin, but Dr. Bashir, Chief O'Brien, Worf and Odo get involved in a bar brawl between the Enterprise crew and a number of Klingons on shore leave. During the brawl, Worf and Odo spot Darvin and bring him back to the Defiant. There, Darvin gloats that he has planted a bomb in a tribble to kill Kirk.

Captain Sisko and Lieutenant Commander Dax infiltrate the Enterprise bridge to scan for the bomb, and confirm it is not on the Enterprise. The rapid breeding of the tribbles makes searching for the bomb on K7 impractical, so they shadow Kirk. They overhear him discussing the tribble infestation, and deduce that the bomb is in the grain storage compartments. They enter the compartments and scan the tribbles, many of which are dead from eating the poisoned grain. Captain Kirk opens the compartment and is covered in falling tribbles. Dax and Sisko find the bomb among the tribbles still in the compartment and the Defiant transports it into space, where it harmlessly explodes. The crew of the Defiant use the Bajoran Orb to return to their time, but not before Sisko takes an opportunity to meet Kirk in person.

The DTI agents, expressing optimism that the crew's actions have not seriously altered history, let Sisko off with just a warning. Once they are gone, Odo summons Sisko to DS9' s Promenade, which is now covered in tribbles, much to the annoyance of Quark.