
In a darkened room, Counselor Troi wakes up with a strained voice ask for the computer to turn on the lights, when it does not respond she rises from her bed and soon finds her surroundings unfamiliar. She makes her way to a nearby mirror and activates the vanity light and becomes startled by her reflection, she's undergone cosmetic surgery to appear Romulan.
She's soon joined by a Romulan sub-commander who informs her of her new identity, a high-ranking member of the Tal Shiar, the Romulan intelligence force, and she's there to ensure a covert operation being carried out by Romulan defectors is able to succeed. As a member of Romulan Intelligence she's got authority over the ship's commander and her being a Starfleet officer may come in handy in the circumstance that the plan isn't able to go off without a hitch.
Meanwhile, the Enterprise picks up a Federation defector who fled to Romulous when he was a young man. He's now re-defected back to the Federation, carrying with him an important message for Picard from Ambassador Spock. The Federation defector was involved with Spock's and other Romulan's underground movement to coax Romulous into reunification with Vulcan and the greater galactic community. He's able to convince Picard of his good intentions and to take the ship to a rendezvous point with an alien freighter carrying important cargo for the defection.
On the Romulan ship, Troi finds out the cargo she's transporting is a trio of defecting Romulan senators being held in stasis, she may be needed as a Starfleet officer should the ship need to sneak into Federation space, using her codes in order to get across the secured border undetected.
Troi conflicts a lot with the warbird's commander who holds a strong distrust of the Tal Shiar and suggests that many other members of the Romulan military likely share similar distrust, suggesting a strong lack of partnership between Romulous' military and intelligence communities.
They rendezvous with an alien freighter intended to carry Troi and the defectors to meet with a Federation ship, but Troi senses deception in the freighter captain and tells the Romulan sub-commander who then destroys the freighter. Troi seizes command over the ship, ordering it to cloak and hold position until a second plan is worked out. Soon the Enterprise shows up looking for the freighter but the warbird is able to escape undetected.
Troi and the sub-commander in private try and to figure out a new plan. The sub-commander wants to go with his Plan B which involves sneaking into Federation space and to deliver the cargo to a destination on their own, Troi wants to use the Enterprise's proximity to carry out the plan sooner but they cannot be tracked while cloaked. Troi asserts her authority over the mission to convince the sub-commander to convince another sympathizer in engineering to create a way for them to be tracked without tipping off the bridge.
The Enterprise is able to track the warbird, as the warbird commander realizes this she intends to attack the ship but Troi takes command of the ship, using the threats of Tal Shiar to assure loyalty from the other officers. She uncloaks the ship and contacts the Enterprise and proposes a meeting between her and Picard to discuss the destroyed freighter.
When communication is ended she says she intends to fire on the ship when the shields are lowered for transport, meanwhile Picard expresses surprise at Troi's image on the viewscreen and has a lock put on her by the transporters.
The Enterprise lowers its shields and Troi orders the warbird to fire. Her co-conspirator fires with a low-intensity disruptor beam that conceals the transport of the defectors. As this information falls out on the warbird bridge the commander re-asserts her command and orders Troi imprisoned. As the warbird cloaks and begins to flee, Troi is beamed back to the Enterprise and has her surgical alterations reversed.
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Through the run of TNG everyone in the cast has their own focal episode where they got the chance to shine. Naturally Picard and Data got these episodes a lot followed by Riker and Worf each dealing with fairly strong themes and strong performances turned in by the actors. For Data it's dealing with humanity, Worf, Klingon issues; Geordi.... I dunno, dating issues?
Troi probably got the fewest episodes that were centered around her and usually dealing with fairly "typical" "female" issues. Not stuff like her dealing with her period or pregnancy but mostly romance, love and stuff along those lines. Somewhat similar when it comes to Crusher-episodes.
This was something of a disservice to Marina Sirtis who is fairly underrated as an actor on the show and wasn't always given a whole lot to do but Season 6 really tried to work to fix all of that and it started with putting her in a proper uniform rather than the bunny suit. Which leaves us with this episode where we get what is easily the strongest of the "Troi Episodes" and a good performance from Sirtis as well.
The episode also ties in good continuity (something Berman often tried to avoid) with previous Trek episodes like the "Unification" episodes dealing with Vulcan/Romulan reunification and Romulan defectors. It also gives us a good glimpse at Romulan "life" and more of a feel for the real distrust there was in the ranks between the military and the intelligence communities. Largely that that was all there seemed to be: Distrust. And the Romulan commander points out that a military ruled by fear and intimidation cannot be very effective and reliable. We also see that, perhaps, Romulans aren't so quick on the draw, considering how much th Romulan laments the destruction of the alien freighter and basically the paperwork and hassle it'd cause. Interesting the Romulans would consider the destruction of a meaningless alien freighter would be a big deal but the commander also seems pretty quick to decide to destroy the Federation flagship, which I assume is a lot more paperwork and, well, a war.
The treatment of the Federation defector (Pudgy McManboobs is what I call him after seeing him in his "civilian" garb once on the ship) is an interesting look at this side of defection but he's also able to quickly regain some level of trust given the news he has. I assume this news is real reason for him returning to the Federation as opposed to the reason he gives Riker which was pretty much, "I dunno... Bored, I guess."
Events of the episode aren't much followed up on since we know hostilities and distrust between the Federation and Romulous continues through DS9 and only just seems to reverse in "Nemesis" which seems to end with perhaps the Romulans and Federation working together and forming something of positive relationship. Though we'll never know. (No! Novels, video-games, and Star Trek '09 don't count!

Good episode that I really like because I think Sirtis does a good job taking the spot-light for once in a fairly a-typical story for Troi's character and at-that for a female-centric story line which more often than not focused on a relationship in some manner. (Looks at pile of booze he has for when "Sub Rosa" comes up.)
Next week: Easily an episode of TNG that's in my top 5 episodes.