The last regular episode before the finale follows the template of the other episodes in the season's home stretch by providing a character story that plays like an epilogue. "Preemptive Strike" is by far the most successful of these episodes, providing the swan song for Ro Laren, a character who never fit the mold of the traditional TNG Starfleet type and who here chooses a path, like Wesley Crusher in "Journey's End," that does not include Starfleet.
Unlike "Journey's End," however, this is a story that grows organically from the character's backstory and benefits from a much better narrative engine (although, notably, the plot involving the Maquis grows directly from what was set up in "Journey's End" as well as DS9's "The Maquis"). Indeed, one of the selling points here is the straightforwardness, even familiarity, of the premise: Ro returns to the Enterprise and is immediately assigned to an undercover mission to infiltrate the Maquis. The story's complexity comes through the choices Ro must make when thrust into a difficult situation where her loyalties become torn. She accepts the mission, uncomfortably, out of loyalty to Picard.
This is a TNG episode that plays on what already at the time had become primarily DS9's turf — with moral ambiguity and political situations involving the Federation/Bajoran/Cardassian demilitarized zone and the Maquis' acts of terrorism. Ro infiltrates a Maquis cell with the perfect cover story, one that speaks to both the appropriateness of Starfleet selecting her for the mission and also the aptness of the script. The truth here is in the details: Ro is infiltrating an organization that includes a good number of Bajorans, and the leader of the cell, a man named Macias (John Franklyn-Robbins), takes Ro under his wing and forms a close paternal bond with her.
The mechanics of the plot are well-oiled and credible without drawing undue attention to themselves: Ro's cleverly covert raid on the Enterprise for the medical supplies allows her to prove her loyalty and usefulness to the Maquis while showing us that Picard understands her tactics in trying to gain the Maquis' trust. But along the way, there's a gradual yet unmistakable shift; Ro becomes so sympathetic to the people she's infiltrating that she realizes her mission, in her own heart, will require her to betray them. (Perhaps the point here is that it would feel like less of a betrayal if she believed in what she's doing.) Michelle Forbes draws us into Ro's plight with a performance that conveys below-the-surface agony in every scene where she has to deceive someone.
Meanwhile, Picard sees an opportunity to cripple the entire Maquis movement when Ro discloses that the Maquis are particularly afraid of the Cardassians developing biogenic weapons. He uses this information to set a trap with the perfect bait, and he uses Ro to plant the false intelligence. But for Ro the situation becomes untenable after Macias is killed in a Cardassian assault. Picard (and by extension, the script) is smart; he realizes that Ro has been emotionally compromised and sends Riker in with her to make sure she carries out her mission, along with a stern warning not to betray her uniform. (He can't pull Ro out of the field now because he needs her relationship with the Maquis to make the plan work.)
Ultimately, "Preemptive Strike" is about Ro's decision at the end, where she torpedoes the undercover mission, betrays Starfleet and Picard, and joins the Maquis. For TNG, it's a fairly radical development (such things would become more commonplace on DS9). It's enough to make you re-examine the episode's gray areas and see that with the Maquis situation the Federation has a complicated quagmire on its hands rather than an easily solvable problem where everyone can be appeased or an enemy can be confronted head-on.
The episode's final shot, after Riker briefs Picard on Ro's betrayal, is one of the most memorable shots in the series. Patrick Stewart's grim-faced silence is more effective than any dialogue possibly could be.