The Invincible begins with a warship called the Invincible in search of Condor, another sister ship. During the investigation of a planet called Regis III, the crew discovers a form of life that is sub-organic (or maybe super-organic, depending on whether you're rooting for the humans or the robots). These lifeforms are self-replicating machines of an insect nature. They're about the same size as an insect, so they're referred to as micromachines, and they have a self-organizing ability that allows them to swarm as a hive-mind, which is a terrifying, nearly prophetic prediction of the ideas of an internet.
The question of what constitutes life is at the center of the plot. The robots are electromagnetic biology, and they can use their senses to do things like detect predators and fight or flee when threatened. They have the ability to self-replicate, the vestige of an alien life form, perhaps. There are robot insects and robot plant life, indicating a robotic evolutionary timeline.
To the human characters, these might as well be demons. They dub the realm of bionic life, "the necrosphere," and suddenly realize that in order to rid the universe of these rival lifeforms, there will need to be some sort of catastrophe, like a nuclear holocaust, picking up the talk of the Cold War. Remember, the book was written during the height of the Cold War, in Poland, right next to Russia, so it's not surprising that the nuclear question should arise from a novella like this.
