After destroying an asteroid on a collision course with the moon, an even larger obstacle appears - an entire planet! On a recon mission, John Koenig encounters an inhabitant who convinces him- on the basis of faith- that the moon and the planet must collide. But no one else supports Koenig's stand.
Moonbase Alpha is attacked and devastated by warships from a nearby planet. With half of its population dead Alpha can no longer survive. Commander Koenig and Dr. Russel go to the alien world to plead for the survivors, but the inhabitants wish no one to disturb their serene existence.
The Alphans encounter the Voyager space probe that has been wandering the cosmos propelled by the Queller drive- a faulty engine system. Following the probe is a fleet of alien ships from a world devastated by the drive.
The Alphans' joy turns to horror when the first child born on the Alpha matures into a five year old in a matter of hours. This is only a prelude to the arrival of formless alien fugitives- who plan to inhabit Alpha's population.
Discredited eagle pilot Tony Cellini is beset by recurring nightmares in which he was the only survivor of disastrous space mission. Now the moon is approaching the same set of circumstances that Cellini encountered.
The moon is approaching a black hole. Professor Bergman is a able to rig up an energy shield around the moonbase, but Commander Koenig feels this is only a desperate measure and has an eagle with six personnel dispatched as the rest of the moonbase await certain death.
Contact is made with the planet Piri, where the main computer created an idyllic life that led to extinction of its population through apathy. Now John Koenig must fight to save Alpha as his people fall under the same fate.