With war heating up between the Federation and the Klingon Empire, Sisko, Worf, O'Brien, and Odo undertake a risky mission to the heart of Klingon space to expose Gowron as a changeling.
- Call Back: Ensign Vilix'pran is mentioned to be having another litter of kids. He also apparently got promoted to lieutenant in the span between episodes.
- Dressing as the Enemy: Sisko, O'Brien, and Odo are surgically altered as Klingons. Even Worf has his forehead ridges altered.
- Evil Cannot Comprehend Good: Changeling!Martok's refusal to challenge Gowron to honorable combat, and his confusion over why Worf is fighting him with a bat'leth instead of just shooting him allows Odo to figure him out.
- Large Ham: Klingon!Sisko really goes to town. After all, a Klingon party is a World of Ham.
- Laser-Guided Karma: One of the Klingons boasts how he recently killed a Benzenite who happened to be an Academy friend of Sisko, who then introduces the Klingon to the mighty Sisko Fist (under the guise that he was blocking the bloodwine).
- No Kill Like Overkill: When Martok is exposed as a Changeling, the gathered Klingons pop him so full of laser fire that he explodes.
- Out-of-Character Alert: How Odo identifies Martok as the Changeling.
- Pragmatic Villainy: When Dukat arrives at Klingon headquarters to drop them off, he tells Sisko he's not sticking around (especially not with his holofilter malfunctioning). He then points out that if Sisko is successful, then the war will be over and they won't need him to return to DS9. If not, then they'll be executed as spies before Dukat can help them.
- Reality Subtext: While discussing having to carry the O'Brien's child to term, Kira says to Bashir "This is still your fault." In real life, the baby was Nana Visitor and Alexander Siddig's own.
- Stunned Silence: Kira screws with Dukat pretty good when she tells him she's pregnant with O'Brien's kid. Making sure to purposely leave out the bit about having it transferred from Keiko. Poor Dukat is at a rare loss for words.
- Taking You with Me: The Martok Changeling attempts to do this with Odo, realizing that the Klingons don't really pay attention to "innocent until proven guilty" and so would be in for a very painful questioning at this accusation.
- Unfinished, Untested, Used Anyway: Subverted—they never get a chance to field-test the polaron gizmos before they're destroyed.
- Unspoken Plan Guarantee: Sisko and his staff discuss their plan early in the episode. It hits a few minor snags before completely falling apart.
- Wham Line: "Martok's the changeling!"
- What Could Have Been: Originally, Dax was supposed to be on the infiltration mission as well, as she's one of the few crew members intimately familiar with Klingon society. However, Terry Farrell got a rash from the makeup and so could not partake.
- You Look Familiar: In-universe—Changeling!Martok almost recognizes O'Brien under his Klingon makeup, and then successfully recognizes Sisko.