Just as a Federation colony is getting settled on a lush planet, the deadly crystalline entity — which consumes life as a food source — shows up and turns the entire world into a barren wasteland. The woman who would've been Riker's next conquest is among the colonists who are killed in the attack. The Enterprise is assigned to pursue the crystalline entity and attempt to make contact if possible — or destroy it if necessary. They take on Dr. Kila Marr (Ellen Geer), a scientist and expert on the entity whose son was killed many years ago in the entity's attack on Omicron Theta (see "Datalore").
There's a moment in "Silicon Avatar" that stands out for me: Riker has just privately made the case to Picard that the crystalline entity is a dangerous scourge that has already killed thousands, and perhaps the Enterprise's mission should be to destroy it to stop its killing spree instead of attempting to communicate with it. After the conversation, Riker leaves and Picard sits there ambiguously. Patrick Stewart's expression shows thoughtfulness without revealing what he's thinking. Does he disagree with Riker? Is he worried he may be right? It's a perfect performance that shows a man just thinking about what he has heard, and processing it carefully. Stewart makes such a brief, subtle moment so memorable.
But I also think this scene echoes my feelings about the episode in general, which is: I'm just not sure what I think about all this. Picard uses an analogy that compares the crystalline entity to a feeding whale, suggesting that it's a force of nature that has as much right to exist as anything else. Fair enough, but we're not talking about shellfish being consumed in mass quantities; we're talking about people's lives and entire M-class worlds being laid to waste. At some point, a line must be drawn. The episode acknowledges this question without quite dealing with it.
Then there's Dr. Marr, whose arc in this story is a little heavy-handed. At first she distrusts Data because his brother Lore betrayed Omicron Theta, leading to her son's death. Later, because Data holds the memories of her son, she weeps as Data reads a letter in her son's voice. Ultimately, she makes the unilateral decision to destroy the crystalline entity by turning the communication frequency into a weapon. I'm not fond of the contrived way she seems to instantly flip from rational to crazy as her revenge brings about "peace" for her son's memory. And the final scene with Data underlines the tragedy in a way that simultaneously feels too conveniently tidy and yet weirdly incomplete.