The Dominion is occupying DS9 and Odo is needed to help with the mine field keeping the Jem'Hadar from coming through the wormhole. This is because Sisko is tired of waiting--he wants to re-take the station and kick Dominion/Cardassian butt. But, with Odo apparently going native and spending his time 'joining' with the other changeling currently on the station, the plan sure looks like it will fall to pieces.
This episode, aside from Odo being a weasel, is really good. There is lots of action, explosions and the like and you cannot possibly be bored by this one. However, you'll need to watch the next episode to see what's next. Will they retake the station and will Odo STOP focusing on joining to the exclusion of everything else?