Troi wakes up aboard a Romulan warbird, looks in the mirror, and sees that she's been surgically transformed into a Romulan. The ship's first officer, N'Vek (Scott MacDonald), tells her that she's been brought on board for a very important mission, and that she must immediately prepare to meet the ship's commander while posing as an operative of the Tal Shiar, the extremely feared Romulan intelligence agency. But no time for questions right now! The commander is waiting! Explain later!
To say Troi is thrust into a situation for which she's not prepared would be an understatement. She's able to survive her first encounter with Commander Toreth (Carolyn Seymour) by bluffing her way through the situation, but she's mostly saved by the fact that everyone so fears the Tal Shiar that she doesn't have to volunteer any explanations. Meanwhile, aboard the Enterprise, the crew takes into custody a former Starfleet officer, Stefan DeSeve (Barry Lynch), who defected to the Romulans many years ago and now has surrendered himself back to Starfleet (older and wiser, the story observantly suggests) but has a message from Ambassador Spock from the Romulan underground, imploring the Enterprise to make a rendezvous to intercept some crucial cargo.
Well, 2+2=4 (unless it's Orwell or "Chain of Command, Part II"), so these two threads must inevitably come together. The plot is a solid exercise of Romulan political intrigue, following up the events from "Unification," albeit without having anything in terms of meaningful or lasting significance. Troi has been brought aboard by N'Vek to ultimately help him smuggle some Romulan dissidents, who are in stasis in cargo containers aboard the warbird, to the Federation. The fact that Troi's an empath was apparently the selling point for why she would've been a candidate for this mission instead of someone more suited to espionage. (That, and we need a decent Troi episode now and then, and the Romulans tend to have a more female-centric authority base compared to most Trek alien societies.)
While "Face of the Enemy" is not exactly to Romulans as "Chain of Command" was to Cardassians, this story does offer an intriguing look into their power structure, and I especially liked the tension between Troi and Toreth, which stems from the fact that Toreth has no love for the Tal Shiar, which years ago hauled her father away in the middle of the night to be questioned, never to return. (The Tal Shiar is prone to police-state tactics, where proving guilt is less important than suspecting it.) The fencing between the two of them is interesting, and by being on the inside we can see the Romulans with a little more subtlety; note how Toreth lambastes the Tal Shiar's overly aggressive tactics, which creates messes, Toreth argues, that the military must then clean up.
The episode culminates with some tactical cat-and-mouse games between the cloaked warbird, the Enterprise, and undercover Troi, doing her best to help get the dissidents transported to the Enterprise. This is well done but not riveting. I should also point out that Marina Sirtis, game as she is here, is not all that convincing when she raises her voice in authority. But "Face of the Enemy" takes a good high-concept premise and milks it for what it's worth and then some.
(Also, yes, Romulan uniforms are still terrible, with a love for shoulder pads that can't be rivaled even by the 1980s, but hey.)