A space probe of immense power and sophisticated technology has apparently annihilated the entire Malurian civilization, and now threatens Enterprise. But when Kirk hails it, the attacks cease and it allows itself to come aboard. Soon enough, the reason for its forebearance is revealed: it mistakening believes Kirk is its creator, Jackson Roykirk. This probe is Nomad, launched from Earth long ago. But its technology is far beyond what was understood then; beyond even what the modern Federation is capable of. What happened to Nomad, and why does it destroy whole civilizations?

Gamma Trianguli VI is a peaceful planet with abundant plant life. It seems a paradise, but the landing party soon feels the thorns: poisonous plants, killer storms, and explosive rocks all claim lives. Eventually, they discover a small community of primitive people who serve the mysterious Vaal, an immense statue, chiefly by feeding it when it demands this. These people live immensely long lives, and do not age or procreate unless there is an accident. Theirs is a stagnant society, and into the bargain Vaal apparently has the means to attack the Enterprise...