Ah, at last, here's the most absolutely necessary episode of TNG's second season. Q forces Picard to hear his request to join the Enterprise crew as a guide. In a wonderful dialog scene that gets to the heart of the human drive for learning by personal experience, Picard refuses on the grounds that Q's presence would defeat the purpose of exploration. (That, and no one likes Q anyway.) To prove his point with a twist of the knife, Q hurls the Enterprise into an unexplored part of the galaxy (two years away from the nearest Federation outpost), bringing the Enterprise into contact with a cybernetic alien species called the Borg. (The episode also implies that the Borg were responsible for the destroyed colonies along the Romulan Neutral Zone.)
The best aspect of "Q Who" is its ability to mix the intellectual with the visceral. In other words, it's the best kind of TNG action show, and should stand as a lesson to sci-fi shows that are action-oriented: Your action works only if it grows from a point of emotion, in this case genuine scariness. The Borg are scary precisely because they cannot be reasoned with and because their technology is vastly superior to the Enterprise's — and those two avenues are the basis by which nearly all TNG stories are typically solved. The Borg have often been described simply as "implacable," and I agree that that's the best adjective for them. They are an implacable foe, and we learn that very quickly by their behavior in this episode.
The industrial-cube design of the Borg vessel is brilliant in its simplicity: Here's a society that has no regard for style or aesthetics but simply raw function. When they communicate, it's with terse directives; they epitomize the laconic. The episode puts good use to Guinan by revealing that not only has she had past dealings with Q, but that her people's world was destroyed by the Borg, essentially turning them into nomads.
Because this is an episode of TNG, the crew is still genuinely curious about the Borg, as are we. An away team beams over to the Borg ship and we get a chance to see their hive-like society, with imaginative visuals and production design. The "Borg nursery" is an intriguingly chilling detail. Such ominous concepts are all the more interesting to ponder when considering the presence of the young and naïve, evidenced here by the cute and plucky Ensign Sonya Gomez (Lycia Naff), whose infectious drive to do her part as a member of the Enterprise crew is met here only with danger. If the show had truly wanted to punch us in the stomach with its dark ambitions, it would've had Gomez die.
The episode plays by its rules. The Borg are a superior and implacable enemy, period, and the only way out is through Q, to whom Picard makes an urgent plea for help when there are no other options. Q sums it up nicely when he says, "It's not safe out here." Indeed, and it's nice to be reminded of that by an episode that is equally as visceral as it is curious, and all but promises that the Borg will be coming for us. If ever an episode deserved to be saved for a season finale in a season that didn't have an adequate (or even tolerable) finale, it's this one.