A couple of dour-looking chaps from the temporal investigation bunch come aboard the station to debrief Sisko on why he took the Defiant back in time. Sisko says they were returning a Bajoran orb from the Cardassians, as well as a human merchant. They passed through a lightshow only to find themselves 200 light years from their previous position. After detecting someone beaming off, they got the external sensors online to identify a nearby ship as the Enterprise (TOS version).
As they worked out, the human merchant was a disguised Klingon, who had previously been on the station which they're now approaching. They determined that they need pop over and search the Enterprise and space station, which involved dressing up in TOS uniforms and Bashir making his haircut crappy.
The Enterprise looks better in this than it does in the "Remastered" versions of the TOS series. Are they still using models at this point?
O'Brien has trouble working the turbolifts. Sisko and Dax work on scanning the ship, while Dax has a bout of nostalgia over the design of the tricorders. Odo wanders into the bar on the station and gets info from one of the waitresses. Over at the bar, Uhura is having an encounter with a Tribble.
O'Brien boggles at the repair work on the ship. He and Bashir are interrupted by an engineer who's confused by Bashir using a medical tricorder. Worf meets Odo in the bar, where Odo shows him a tribble, which naturally has a bit of a reaction to Worf. Odo is confused by their respective reactions, and Worf says that the Klingon empire wiped them out years ago.
There's a red alert, and Sisko contacts the Defiant - a Klingon battleship has dropped out of warp. Dax recognises it as Koloth's ship. She's keen to see her old friend, but Sisko sends Bashir and O'Brien over. The pair pop into a turbolift, where Bashir is hit on by a crewwoman - who Bashir realises may be his great grandmother and gets into a philosophical argument with O'Brien.
Sisko and Dax spot Kirk and Spock making a call to the station. Dax eyes up Spock, and is surprised that Sisko doesn't want to meet Kirk.
Bashir and O'Brien meed Odo and Worf in the bar. Scotty, Chekov and another crewman wander in. The crewmen from the future are surprised by the appearance of TOS Klingons, and Worf simply says it's a long story. Over at the other table, the TOS crewmen get into a fight with a Klingon which erupts into a bar brawl. O'Brien and Bashir are grabbed by Starfleet security.
Back in the future, the TI people complain about Sisko's men taking part in the fight. Back in the flashback, Kirk confines everyone from the fight to quarters. O'Brien and Bashir stumble on the tribble problem. Meanwhile, Worf and Odo have caught the senior Darvin, who's made an exploding tribble. Sisko and Dax decide they need to go to the Enterprise bridge to scan the ship for explosives. There they spot Kirk again, who's somewhat annoyed at the abundance of tribbles. Dax remembers meeting McCoy when he was a medical student and implies there was a relationship. They complete the scan, determining the bomb isn't on the ship.
O'Brien, Odo and Bashir scan tribbles in the bar, but the tribbles are multiplying too fast. Dax reasons if they stick close to Kirk, he may lead them to the exploding tribble. They follow him into the mess hall and, after Kirk mentions the storage compartments on the station, the pair start scanning the tribbles in the grain compartments. Finally Sisko finds the exploding one and has Kira beam it into space.
Sisko admits that he did go and have a chat with Kirk before they returned to their own time. Satisfied, the TI guys leave for the docking port. Then Sisko goes down to deal with the tribble infestation they've brought back with them.