“The Search: Part 1” is episode one of season three of Star Trek: Deep Space Nine.
In the season two finale, “The Jem’Hadar”, the Federation had finally encountered the true forces of the Dominion. The result of this was the destruction of a Galaxy-class starship, the USS Odyssey. Galaxy-class starships are Starfleet’s most powerful vessels, but its shields proved ineffective against the Dominion’s weapons and it was destroyed when a Jem’Hadar warship rammed it.
Commander Sisko returns to DS9 from earth, where he has picked up a new vessel, the USS Defiant. The Defiant was the first vessel in a purely warship class – not something the Federation builds – intended to fight the Borg. The ship had flaws in its design too, which is why it was never used. It also has a borrowed Romulan cloaking device, and a Romulan on board to operate it (curious that a Klingon one wasn’t used instead).
Its mission is to go the the Gamma Quadrant and try to find the Founders – the legendary people who run the Dominion. Quark and Odo also accompany them, Quark because he has had business dealings with Dominion planets. Odo sees things in the Gamma Quadrant that look familiar.
The story continues in “The Search: Part 2”.