This episode is known in my household as the "Ugly bags of mostly water" episode, a line I actually remember from the first time I saw it. Kudos to Data for teaching me that humans are 90% water, and before I was old enough to attend school. Talk about a lasting impression!
The Enterprise stops by Velara III to check on the terraforming crew. They all seem fine, except that Counselor Troi can sense that Mandl is hiding something. Picard insists on sending down an away team as visitors, and while they are there, one of the terraforming group is killed by a psycho drilling laser. When Data and La Forge are checking out the vicinity for clues, they find a glowing speck which they take to the Enterprise. The speck is inorganic, yet appears to be alive - this becomes a certainty when it not only speaks to them through the ship's translator, but also self-replicates. Picard then has a stern talking to the terraformers because some of them kinda sorta knew that there might be life on the planet. The speck becomes a ball and declares war on humans for destroying it's habitat. By this time, the alien has taken over the remote controls to the lab. The Enterprise then figures out that the alien subsists on light, so they dim the lights to the lab manually. Everyone reaches a truce and the aliens are returned home with the promise that humans won't bother them for a while.
This may not be the most amazing episode ever, but the premise appeals to me. As a scifi writer, a non-humanoid alien can be fun to write. An inorganic alien that communicates with patterns in the sand? Heck yes.
Of course, what really takes the cake for this episode is the "ugly bags of mostly water" scene. When the aliens get the translator really working, that's the first thing they say. Picard is baffled, which is always fun to see, and Data has to explain it to him, which is also always fun to see.
I have to say though that this isn't an episode you take seriously. It may be amusing, but you never really feel like the characters are in danger, even though the alien is taking over the ship. None of the characters really stand out, either - oh! Unless you count Data kicking the butt of the psycho laser. Yeeeeeees. Is that not the point of having a super-human android?
This episode was on the predictable side, but I still enjoyed it. It taught me science when I was an impressionable preschooler! Win!