Sisko tells her that without proof, she can't do anything about it. Before it goes any further, their attention turns to a Klingon ship returning through the wormhole. A month ahead of schedule. And it promptly explodes.
O'Brien manages to beam one Klingon, Hon'Tihl aboard. But he can't be saved. He speaks one word before he dies: "Victory."
Hon'Tihl's ship was on a simple bio-survey mission. Which makes the weapons burns on his body and his cry of "Victory" rather unusual. Needing to get to the bottom of things, Dax and O'Brien head off in search of the ships mission recorder.
Odo drops by Quark's for his usual round of blackmail-assisted info gathering. Quark mentions the crew, who trashed several holo-suite rooms by the way, spoke of something that would "make the enemies of the Klingon Empire tremble." That's all he has.
As Odo leaves, he's racked by a mysterious force. His head morphs oddly as if it's shape-shifting of its own accord, and nearly splits in two before returning to normal, and so Quark goes hurrying off to find Bashir.
Awakening in sickbay, Odo appears no worse for wear. Which is good, because Bashir has zero clue about how his body works. He prepares to leave, only to be stopped by Bashir, who starts talking tough in a manner that is very out of character, and questioning Odo about an "Uneasy alliance" aboard DS9.
Bashir isn't the only one acting funny. Dax has gone spacey, randomly recounting old stories out of the blue. Kira has become a vicious conniver, looking to topple Sisko and O'Brien. Sisko has become an aloof and paranoid man, with O'Brien as his threatening right hand man.
With the mission recorder recovered, O'Brien sets to work decrypting the journal logs, while Sisko holes up in his room building a clock.
Odo is finally forced into action when Quark comes shuffling into his office, stuck in a neck-brace. While tending the bar, he overheard Kira trying to sway Dax onto her side in an attempted coup. Prompting Kira to threaten him into silence, and toss him like a ragdoll.
Examining the logs, Odo finds out the Klingon ship was destroyed from within. The crew was possessed by an extinct race known as the Saltah'na when they stumbled upon their mysterious energy spheres. The Saltah'na wound up destroying themselves in an internal conflict. And they've picked up right where they've left off in new bodies.
Working with Bashir to find a solution, Kira makes her coup attempt. It fails, and Sisko and O'Brien try beaming onto the Valerian ship. It's a no-go, and they only get transported into a corridor.
Odo is playing both sides of the conflict, herding all the possessed people into a cargo bay. With a specially designed program, he expels the Saltah'na from the crew, and jettisons it out the nearest airlock.
With that, things resume as normal aboard the station and Sisko finishes his clock, placing it as a permanent fixture in his office.
- Almost Dead Guy: Hon-Tihl, who beams onto DS9 after his ship explodes and just before he himself dies of his injuries.
- Apocalyptic Log: The journal of First Officer Hon-Tihl details how the Klingon expedition came into contact with the records of the extinct Saltah'na, leading to mutiny aboard the Toh'Kaht and its destruction.
- Attention Deficit... Ooh, Shiny!: Dax under the influence of the Saltah'na. Also Sisko, to a lesser degree.
- Badass BoastSisko: You know what disturbs me? The ingratitude. I offered you my kindness, my help, my leadership, and how do you repay me? With betrayal. But you won't get rid of me so easily. You see, unlike you, I understand history. My name will blaze across the stars long after your petty treacheries have been forgotten.
- Bizarre Alien Biology: Odo is not affected by the energy field because of this. When it settles on him, his head goopifies, appears to split, and he collapses.
- The Caligula: Implied in Sisko's persona, who Mood Whiplashes between indifference to matters of state, to sudden outbursts of violence and Large Ham.
- Casual Kink: Odo is discussing the Klingons with Quark, who says, "Spare me from beings who think pain is pleasure. [beat] In small doses, perhaps..."
- Conflict Ball: In-Universe. Though there is still some lingering distrust between Starfleet and the Bajoran Militia, it is only because of the telepathic matrix that they start scheming against each other.
- Conveniently Timed Attack from Behind: On several occasions someone is nearly murdered, but is saved by the intervention of a third party.
- Cut Comm Channels: Odo tries to let higher authority know what's going on, only to find the Subspace Ansible to Starfleet has been shut down under Kira's authority, while communications with the Bajoran Provisional Government have been cut off under O'Brien's authority.
- The Ditz: Dax, who appears to be channeling the persona of a senile old man, easily dominated by the forceful Kira.
- Double Entendre: Odo's first sign that the senior staff are acting strangely is when Bashir and Kira start smirking and talking in political double-speak.
- Dragon-in-Chief: O'Brien, who runs the station while Sisko is locked up in his quarters, working on his hobby.
- Dramatic Drop: Kira says that she's going to remove Sisko "one way or another". Cue Off Screen Crash as Quark drops a tray, tipping off Kira that he's listening in on their conversation.
- Easily Forgiven: Kira's attempted mutiny, since she wasn't herself at the time.
- Even Evil Has Standards: One of the things Odo uses to convince Bashir to help him cure the virus (since he refuses to believe that he has been affected) is the possibility that if any of the others were affected they could destroy the whole station in their madness - though it quickly pales under the other point that he could use the cure to control everyone.
- Evil Is Hammy: Sisko is probably the worst offender, but everyone has their moments. Quite frankly it's a wonder they didn't have to build a whole new set after this episode, everyone was chewing the scenery so hard.
- Guile Hero: Once Odo works out what's going on, he uses everyone's ambition and paranoia to get them into one place where the virus can be purged from their heads.
- He's Dead, Jim: Bashir re Hon'Tihl.
- Ice-Cream Koan
- Idiot Ball: A minor moment of stupid at the climax: Odo tells everybody to grab onto something secure before he opens the cargo bay door. All the other characters grab cargo containers, i.e. the only things in the bay guaranteed not to be secure. Fortunately, the narrative is on their side and those containers were apparently filled with very heavy cargo.
- Most of the cargo was visibly strapped to the floor, though not the one Sisko grabbed.
- Implausible Deniability: Odo suggests that the Ops staff may have been infected with a telepathic virus. Bashir acknowledges the possibility, but says it's unlikely as HE certainly isn't acting any different.
- Indirect Kiss: Kira drinks from Dax's Modela aperitif.
- Jack Bauer Interrogation Technique: Sisko tries this on a Bajoran Kira sent to assassinate him, but he's interrupted by Kira before he can extract the information and/or kill the Bajoran.
- Last-Second Word Swap: Bashir is handing an injector filled with poison to one of Kira's men, to be used in Sisko's assassination."It'll take effect in two minutes. Plenty of time to get a good— (Odo enters) Plenty of time for you to get a good night's sleep."
- Leaning on the Furniture: Odo enters his office to find Kira sitting in his chair, with her boots up on his desk.
- MacGuffin: The Valerians only exist as the Conflict Ball between Sisko and Kira, never reappearing in the series.
- The Mutiny: Though those concerned are reenacting The Coup and act accordingly.
- Not Himself: Everyone except Odo and Quark.
- Only Sane Man: Odo is the only one in Ops not affected by the Saltah'na.
- Out-of-Character Alert: And it only takes Quark getting assaulted for Odo to finally acknowledge it.
- Pet the Dog: Despite Odo having just blackmailed him, Quark is horrified by the sight him uncontrollably shapeshifting and instantly runs to get Bashir.
- Playing Both Sides: How Odo manages to solve the whole problem, he convinces Bashir that removing the psychic interference would give him power over the rest and then pretends to ally himself with both Kira and Sisko in order to lead everyone to the same place.
- "The Reason You Suck" Speech: Sisko gives one to Kira about her ingratitude.
- Somebody Else's Problem: Sisko ignores a brewing mutiny to construct and build a clock.
- Standard Evil Empire Hierarchy: The Mind Virus forces those affected into defined roles.
- The Emperor: Sisko; likely The Caligula type, with more interest in his hobbies than doing his job. This leaves the field open for whatever ambitious underling wants to try for the top job.
- The Right Hand: Chief O'Brien, also doubling as the Security Officer (not Odo, as he's not affected) given his desire to root out traitors.
- General Failure: Dax; though implied to be long past 'his' prime. Kira is likely channeling a young and ambitious officer under his command.
- The Oddball: Bashir; eagerly waiting to see which side comes out on top and secretly aiding whoever he thinks will benefit him.
- The Starscream: Kira.
- Suddenly SHOUTING!:
- Taking You with Me: The Klingon captain destroys his own ship and everyone on it. Clearly Odo's concern about someone destroying the station is not exaggerated.
- Thrown Out the Airlock: Once Techno Babble is used to force the telepathic virus out of everyone, Odo tells them to hold on and briefly opens the cargo bay door to space, where it disperses.
- Vitriolic Best Buds: When Odo collapses right after blackmailing Quark into providing information, the Ferengi rushes off shouting for Dr Bashir.
- What Happened to the Mouse?: We never find out what became of the Valerians, or what Sisko told them about what happened on the station.
- Wild Card: Bashir eagerly observes the impending crisis on the station, waiting to throw in his lot with whoever looks like winning. Odo gets his loyalty by convincing him he can win in his own name.
- With Us or Against UsO'BRIEN: Anyone who is against Sisko is against me.
- Would Hit a Girl: O'Brien cuffs Dax when she tries to signal Kira.
- Wounded Gazelle Gambit: It's subtlety implied that Quark is faking his neck injury in order to sue Kira. When Odo walks out without acknowledging his desire to press charges, he moves much faster and yells much louder than his previous actions would imply he was capable of and even pauses to adjust his neck brace when no one is looking.