The Adversary: The Defiant sets out on a rescue mission, but there's a saboteur on board...
Sisko's getting a promotion to Captain, and the staff's having a party to celebrate. He's taken aside by an ambassador who says there's been a coup on the Tzenkethi homeworld which they need to deal with. So after the party, they start getting the Defiant ready for launch. O'Brien, while checking out the engine room, hears an unexplained thump.
Dax asks if Sisko's told Captain Yates about his promotion, and he says he barely had time to contact his father. She probes further about how the relationship is going, and he says he's going to take her to the holosuite to watch a ball game. Awwww.
The Defiant sets out for the Tzenkethi border. In a Jefferies tube, O'Brien hears more thumping noises and finds Bashir, who says he was hooking up his medical console. Sisko tells Eddington to make sure the Admiral accompanying them is safe should they be attacked. Gosh, this is all MASSIVELY EXCITING!
They get a distress call from Barisa Prime, which reports they're under attack from Tzenkethi warship. They have trouble with communications so they can't contact Star Fleet - O'Brien and Dax find a parasitic device attached to the systems. They realise someone on the ship is a saboteur. O'Brien admits that he saw Bashir crawling around the tubes. Dax has an idea - the saboteur would have been exposed to tetryon particles when they put a device in the warp core, so Dax starts scanning everyone. Bashir comes up clean - and says he wasn't in the conduit the other day. Dax scans the visiting Admiral, who turns out to be a changeling, and makes with the sodding off.
The cloaking device comes on, and the crew find that they can no longer control the ship. The security teams sweep the ship, but are unable to find the changeling. Sisko speculates that the Founders are trying to provoke a war between the Federation and the Tzenkethi, and that the coup and attack on Barisa Prime were all faked. The changeling is probably disguised as a crewmember, so non-essential crew are locked in their rooms, and the others arm themselves and look for the changeling.
O'Brien finds Dax unconscious in engineering. Sisko says if O'Brien can't get the ship under control before they reach the border, he'll have to destroy the ship.
They set up phaser rifles to a wide-beam setting and start sweeping the ship. Sisko finds it in a Jeffries tube and they start closing in. Sisko finds Kira and her Bolian companion, and there's a paranoid moment when they realise that they don't know who the Changeling is. Then Odo points out that Sisko is bleeding - if he was a changeling, the bloody would have reverted to goo. This leads them to start taking blood samples. WHY HALO THAR THE THING.
Bashir takes samples, and Eddington's sample turns to goo. They go to lock him up, and find Bashir in a cell - the second Bashir is the changeling. Ooops. The bridge reports they're entering Tzenkethi space.
And we still don't know WTF a Tzenkethi even looks like. Nor have we ever heard of them before. Typical. Ah, they'll just be people with stuff on their foreheads.
Kira reports the ship is heading for a Tzenkethi settlement. Sisko starts the self-destruct. O'Brien reports he's worked out a way to fry all the forcefields. As he works, two Odos appear, with no indication as to which is the real one. When O'Brien zaps the forcefields, one of the Odos attacks, but the real one throws him onto the warp core, frying it. The dying changeling whispers something into Odo's ear before turning into a fine powder.
Fortunately O'Brien is able to get the ship under control, and Sisko aborts the self-destruct, and they return to DS9. Sisko says Star Fleet wants reports from everyone. And Odo says the Changeling whispered "You are too late. We are everywhere." Wooo, paranoia!
