On the dark side of the moon, which isn't really dark, lives an otter who isn't really an otter. He was designed to live in the harshest environments in the solar system and to build the solar powered machines that may someday help sustain human life in these places.

His caretaker, Pen is a young genetic engineer who spends each day trying to help him better communicate with the human beings that he will someday be working for. They are making progress. But after losing contact with a ship carrying someone dear to her and believing that this creature may know something about the ship's fate, Pen decides to take drastic steps to finally allow them to truly understand each other. In doing so Pen will come to learn that the small world that she had thought this creature inhabited is not really as small nor as empty as she had imagined.

This otter-who-isn't-really-an-otter was given the name “Sol-4” by the engineers that designed him, but that isn't really his name.