I was very impressed by this episode. For a long while, The Next Generation was my favorite Star Trek series, but at some point I realized that Deep Space Nine had surpassed it. Episodes like this one are the reason why.
Dr. Bashir is recruited by a man named Sloan, a member of a group known as Section 31 that operates outside of Federation Law to advance the Federation's agenda, on a mission to obtain information about their Romulan Allies. Bashir, one of our heroes, is reluctant to help such a man but ends up playing along after some persuasion. Bashir discovers that Sloan is planning to assaninate a high-ranking Romulan and does everything in his power to prevent it. In the end, he comes to realize that he was being manipulated all along.
This episode was very well written, and revealed some harsh truths about Starfleet. Generally, starfleet is portrayed as being very righteous and the audience roots for them all the way. An episode like this shows Starfleet to have a dark side, something some members of the audience may have trouble accepting. But shades of gray are often more interesting than good guys vs. pure evil. This episode makes us question everything we've come to believe about the Federation. To that I can only say: bravo.
This episode is also a nice precurser to some of the darker science fiction series that would follow DS9 (Firefly comes to mind). This new, darker Federation would have been an interesting thing for Star Trek to pursue in a future series, but unfortunately, they opted to do a prequel instead. Maybe someday DS9 will get a proper follow-up.