Synopsis: A human team exploring a planet which emits copies of old radio programs from Earth finds a race of organic beings apparently bioengineered to emit radio signals, but then is attacked by a seemingly dormant ancient alien orbital installation. A few years after this event, Earth becomes mysteriously infested with monstrous sea creatures which attack and destroy any human ships, but also another seemingly friendly species, who appear to be the adults of the destructive creatures, and help a few humans survive both the monsters and the destructive nuclear war that breaks out worldwide. The interstellar probe has now reached another planetary system, where they find signs of life deep in an ocean covered with kilometers of ice, with another ancient alien installation in orbit. A team member with prior experience of alien artifacts on Earth theorizes that an ancient machine intelligence has placed similar weapons around any planet that might develop organic intelligence to destroy it if it appears, the wreckage of one such being what he found on Earth, which allowed humanity to escape destruction; and that the machine aliens reacted to the contact with humans by tracing their origin to Earth and transporting the destructive aliens there. After the alien orbital weapon cripples their vessel, the survivors devise a way to attack and disable it, whereupon they find the original interstellar ship which brought the mechanical aliens to the system, and they resolve to use it to bring the fight to the aliens throughout the galaxy.