The Defiant is returning from Cardassian space with the Bajoran Orb of Time. A Klingon named Arne Darvin, surgically altered to look human, has come aboard as a passenger. He uses the Orb to send the Defiant more than one hundred years into the past — near the original U.S.S. Enterprise as it orbited Deep Space Station K-7. Records reveal Darvin was a spy then, and that he was caught having poisoned a shipment of grain on the station. Darvin is out to change history, possibly by killing Captain James T. Kirk, who originally exposed him as the spy.
While Sisko, Dax, Bashir, and O'Brien dress in period uniforms and search the Enterprise for Darvin, Odo and Worf, disguised as civilians, do the same on the station. Soon they are interacting — albeit as little as possible — with Starfleet legends from a century ago. Odo, at the station's bar, watches as Lt. Uhura has her first encounter with a furry, cooing, adorable little creature known as a Tribble.The Enterprise goes to red alert — a Klingon ship is approaching the station! But they only want shore leave. Later, in the K-7 bar, Bashir and O'Brien join Worf and Odo, and are swept into a brawl between the Klingons and Enterprise crew members, including Mr. Scott and Ensign Chekov. Odo and Worf spot Darvin during the fight, just as Bashir and O'Brien are taken by Enterprise security personnel.
Bashir and O'Brien are questioned by Captain Kirk himself, then soon notice that the Enterprise is littered with Tribbles. Meanwhile, Worf and Odo materialize on board the Defiant with Darvin. He reveals that he planted a bomb in one of the Tribbles, set to kill Kirk within the hour. Sisko and Dax go to the Enterprise Bridge and scan the starship, but find no explosive. That leaves a manual scan aboard K-7, but there are over a million Tribbles involved. Realizing the bomb is in the storage compartments, Sisko and Dax enter the bin where the poisoned grain is stored, and see that the Tribbles have eaten it all and died. They scan the compartment — while Kirk opens the outside door and is hit by a stream of falling Tribbles. Luckily, Sisko finds the explosive Tribble before it finds Kirk and has the Defiant beam it into space, where it detonates harmlessly. The timeline continues uninterrupted, allowing Kirk to expose Darvin as before.