The cold opening has Jake gearing up to say something dramatic, which turns out to be celebrating his dad’s promotion to captain.
During the festivities, Ambassador Krajensky injects some exposition about joining the Defiant on a patrol of the volatile Federation-Tzenkethi border.
In the Defiant engine room, O’Brien hears a noise, but dismisses it as unimportant. Later, he’s surprised to find Bashir crawling through a Jeffries tube, having put his “engineering extension courses to work” connecting some medical equipment.
Sisko starts a new captain’s log. Dax prods him a bit about his relationship with the freighter captain Cassidy Yates.
Recurring character Eddington injects some military-style formality into what has become a fairly chummy group. He promises he won’t let it bother him if the ambassador doesn’t like being escorted off the bridge in a crisis.
We hear an audio message that suggests hostilities have broken out, but a glitch in the Defiant’s technical jimberjam doohickey prevents Sisko from checking in with Starfleet.
Strange devices with organic-looking cables seem to be spreading throughout the ship. Sisko concludes a saboteur is on board, which sets the stage for an old-school shipboard whodunit.
Bashir is a suspect because O’Brien found him poking about the ship’s innards, but a plot-contrivance-particle detector clears him, and the fact Bashir seems not to remember having met O’Brien in the conduits points towards a Changeling infiltrator.
Sisko notes that if they can’t regain control of the ship he’ll have to destroy it. Dax is knocked out, Odo and Eddington debate tactics, and teams start sweeping the ship with phaser rifles that will “affect the changeling” in some unspecified way.
I’ve commented before that DS9 does a very good job with short scenes involving two characters, often discussing a third character who has just left or will shortly arrive, with dialogue that reveals character while also advancing the plot. Those rapid-fire short scenes (which always remind me of the ballroom dance sketches on The Muppet Show include parings like Odo and Eddignton, Kira and a suspicious Bolian, and Sisko and a rando. When some of the pairs get separated, suspects abound.
After Odo mentions a random detail about Changeling physiology that might have been really helpful to know a few episodes back — namely, that when any part of a Changeling leaves its body, that part reverts to its liquid state — it’s time for a blood sample lineup.
After a clever bit of misdirection, and a too-convenient discovery that reveals who the real doppelganger is, Odo chases after the Changeling intruder, and Sisko sets the autodestruct countdown.
Two Odos show up in engineering. O’Brien wisely tells the guards to keep their rifles on both, as he works to unflabbergast the warp core jimberjam.
The other Founder tries to make Odo turn into goo, but Odo makes his opponent turn solid, then throws him against the warp core, just in time for the plot contrivance particles to fry him into a crispy critter, who whispers something to Odo just before dissolving into dust.
In the final infodump, we learn the supposed war on Tzenkethi never happened, and Odo reveals that the dying Changeling whispered to him, “You’re too late. We are everywhere.”
And with that, Season Three ends.
It’s a good episode that has little to do with Deep Space Nine itself — highlighting just how much the show needed a powerful ship like the Defiant to advance the overall story arc.