"Arsenal of Freedom" is like a lot of Star Trek episodes, completely episodic in nature, with our crew stumbling into a problem on an alien planet and subsequently trying to figure out how to make things right. In this instance, as the title suggests, it's all about a defense system that ran horribly awry, killing the original inhabitants of the planet and imperiling the unwitting Starfleet officers who reactivate it. Like a lot of stories in the franchise, the episode in essence demonstrates a deep ambivalence to weapons of war.
Thankfully, it's an episode that you can easily enjoy, just so long as you know exactly that it still represents the first season of Next Generation, generally inferior to the rest of the series (and franchise). "Arsenal" is generally enjoyable, so it's not hard to recommend, at least at a basic level.
The selling points beyond this general interest involve a few of the budding leads. La Forge has a non-character specific spotlight when he's forced to take command when every superior officer is trapped in the quagmire on the planet's surface. LeVar Burton, who was one of the few known commodities at the start of the series thanks to Roots, takes full advantage of the opportunity, and it's a treat to watch him dominate for a change.
Otherwise it's the abortive relationship between Picard and Crusher that helps distinguish the episode. Always a vague element of their backstory as well as their present (they previously served together, until Crusher's husband was killed on an away mission, which strangely never really gave Picard a definitive sense of responsibility over Wesley Crusher) is the romance that never quite happened. In fact, "Arsenal" was pegged to address this more directly, but apparently Gene Roddenberry nixed that idea in the bud. But they still have that shared history, something that already gave Next Generation more continuity than its predecessor, which preferred to keep most personal details strictly private for its characters. To varying levels, this new series featured characters incredibly rich with pasts, especially its captain, notably older than Kirk at the start of our acquaintance with him.
Of course, the entire second season completely ignores Picard and Crusher's relationship, because Crusher has been temporarily replaced, and this can't help but feel like Roddenberry's further attempt to distance Next Generation from what it was already becoming, and what further incarnations enthusiastically embraced. Then again, even having an entirely new set of characters in the Star Trek universe already gave the budding franchise something it didn't have before, a defined history. "Next generation," after all, wasn't just a title, but an indication that Picard followed Kirk and therefore they definitely exist in the same universe.
Anyway, that's what "Arsenal of Freedom" both does and doesn't do. It also features another of this Enterprise's random chief engineers, which makes it pretty ironic that La Forge has one of his most prominent moments to this point in the series, because he would soon enough assume that title. It's still strange to think that he was ever anything but Scotty's successor.
Also of note is Riker's famous quip when he realizes he's talking to a doppelganger, about serving aboard what he calls the USS Lollipop: "It's a good ship." Such sly humor from Number One...
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Memory Alpha summary.