The crew discovers that visiting Klingon officer D'Jan (Henry Woronicz) has been stealing technical secrets from the Enterprise and transmitting them via quiet and clever channels to the Romulans. There also has been an explosion on the ship's warp core, leading D'Jan to become the obvious suspect of sabotage. Admiral Norah Satie (Jean Simmons), a renowned Starfleet prosecutor, comes aboard the ship to aid in the investigation of the matter. "Aid in" quickly becomes "take over," and soon she's presiding over a sprawling paranoid inquiry involving unconfirmed speculation, serious allegations, and public hearings. Picard strenuously objects to what becomes a witch hunt.
It starts small and builds slowly: Just a few questions of a few people. Satie seems to be doing her job, and even I thought Picard was being overly naive when fussing over the fact that her second chair is a Betazoid. But soon the investigation has narrowed in on Crewman Tarses (Spencer Garrett), suspected merely because he worked in sickbay when D'Jan came in for routine procedures. Satie continues to press on, and ultimately presses Picard for not clamping down, despite the lack of incriminating evidence on Tarses and, further, with strong emerging evidence that the explosion in engineering was actually an accident. The details of the episode are solid, but it's the message that really works here. It's painful to watch Tarses destroyed over the mere fact that his grandfather was Romulan (rather than Vulcan, as he claimed). It's presumed guilt by national ancestry.
Ultimately, Picard is called to testify, in what raises the stakes to a witch hunt while, in narrative terms, serves to turn the story into a battle of wills between Satie and Picard. (Hint: Never bet against Picard.) The way Satie twists the facts is deplorable; I liked the story's invocation of continuity where she essentially attacks Picard for being abducted by the Borg.
There were numerous "courtroom episodes" on Trek throughout the years, and "The Drumhead" is one of the best. With the threat of terrorism and the ensuing questions of curtailed individual rights at the forefront of today's sociopolitical discussion, "The Drumhead," like DS9's "Homefront," is even more relevant in America today than when it originally aired. In a way it seems eerily prescient — until you consider that these issues have repeated themselves in cycles as a result of whatever the paranoia of the moment may stem from, whether it was the Japanese during World War II, suspected communists during the Cold War, or terrorist "persons of interest" post-9/11.
"The Drumhead" is a bit theatrical at times; one wonders if Satie, supposedly such a seasoned professional, would so easily be baited into a meltdown at the end. Or that she'd so easily have been able to lull Worf into her camp. But perhaps that's the point: The law has been hijacked by an overzealous individual whose judgment is suspect. (You can insert your own current-day political commentary here.)