Der opstår rædsel i kabinen, da man kommer på en kurs mod en asteroide, der pludselig brænder op og bliver til en alt-opslugende »sort sol«.
Alpha detects a nearby asteroid and monitors it to insure it won't collide with the base. It is determined that it will miss the Moon, but suddenly changes course right towards Alpha. It changes course again, at the last possible moment, and is destroyed by an unknown force.
Astronaut Mike Ryan is sent to investigate the phenomena. His Eagle is pulled into it moments before Bergman's calculations prove it to be a black sun. When he arrives in Main Mission to warn Koenig of the danger, it is too late.
Unbelievably, the Moon has changed course and is being drawn into the black sun, an object with a gravitational field so strong nothing can escape it, not even light. As it slowly saps their power, they must figure a way to survive it's destructive forces.
Bergman comes up with the idea for a forcefield that will use the force of the black sun to protect Alpha. Koenig orders all the atomic engines from the Eagles to be used to power the field generators.
To demonstrate the abilities of the force field, Koenig and Bergman go out in spacesuits, and have Alan fire an Eagles laser at them with only the forcefield to protect them. This display is a success with everyone but Helena.
Koenig explains to her that the test was only to boost morale, that realistically, their chances of surviving the black sun were slim. He also tells her that he saved one Eagle to use as a life boat, so that a select few could survive.
After the computer selects the team with the highest probability of survival, the Eagle is launched, and heads away from Alpha, the black sun, and danger. Among the team are Alan, Helena and Sandra.
With only enough power left to run the forcefield, the rest of the Alphans have little to do but await their fates. As Koenig and bergman sit in an empty Main Mission, they become transparent when the Moon enters the black sun.
They age suddenly and communicate telepathically as their awarenes of the universe grows. They encounter a being (God?) who thinks a thought every thousand years, but before they can talk at length, they have passed through the black sun unharmed.
As amazed as the the Alphans are at their survival, their thoughts are now with the comrades they sent away. How could they possibly hope to ever see them again?
But somehow (perhaps the intervention of the cosmic intelligence they encountered in the black sun?) the Eagle experienced a similar effect to the one witnessed on Alpha, and returns to a delighted base.
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