Invaders is about events that occur several years after those in the novel, Disc.

GSI (Gettnor Space Industries) is beginning to investigate our solar system and mine the asteroids. Aircraft and automobile companies are beginning to switch over to thrusters from their more traditional products. New uses for thrusters are popping up in the health care and sports industries.

However, at Epsilon Eridani, a race of aliens has so overpopulated their own solar system that they must move many tens of billions of their people to another system before overcrowding destroys them. They send an exploratory vessel to our solar system to determine whether it would be a suitable place to move. The mere presence of another intelligent race doesn’t deter them—they’ve always just wiped out any problematic species in new systems.

When the aliens first arrive through a wormhole near the Sun, their ship is thought to be an asteroid in a highly elliptical orbit. Earth isn’t even aware that an alien race has arrived until the aliens separate a lander and send it to study Earth.

Attempts to communicate with the aliens fail. The aliens’ complete lack of any interest in communication with us contributes to these failures. The aliens are far ahead of us in biotechnology, materials technology, and wormhole physics. They aren’t really worried about any threats from the human race, but they don’t recognize that we have advantages in computing and thruster technology.

Will the aliens be able to successfully deploy their bioweapon, destroying the human race and moving their excess population to our solar system? Or can we fight back...somehow?