Plot

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.—Anonymous

A fair-sized asteroid is passing by the moon's course, but reports indicate it will miss the Moonbase. Suddenly, Alpha's sensors trigger a collision alarm as the asteroid's direction had changed, as though it had been pulled off its course. With less than a minute before it'll impact right into the heart of Alpha, the asteroid changes course again, moving away from them. The asteroid then begins twisting out of shape, followed by it ceasing to exist. Paul Morrow (Prentis Hancock) informs Commander Koenig (Martin Landau) that sensors indicate the moon is now changing course. A view on the Big Screen is focused on something ahead of them on their new course; an enormous circular black entity with refracted light from behind it surrounding its rim.

Professor Bergman (Barry Morse) has retreated to his quarters to try and calculate the identity of this unusual space entity, with help from computer.

In the meantime, Commander Koenig has sent an Eagle out ahead of them to try and gain information. Pilot Mike Ryan (Paul Jones) - who's been seeing Sandra (Zienia Merton) - arrives at the point where the asteroid was first pulled off its course. He sends multiple radio signals toward the entity, but nothing's bouncing back; the signals are being absorbed by the huge black mass. As Ryan's holding his position, visual communication with the ship's pilot section is becoming covered with static. No fault is registering; it's as though the ship's power is being drawn. Ryan fires a laser beam into the object and that becomes absorbed by it as well. Against Koenig's advice, Ryan moves in closer to the object; determined to learn what it is, as Alpha is headed right for it. As Professor Bergman returns to Main Mission to report to Commander Koenig that he's discovered the object's identity, he arrives in time to witness Ryan's Eagle on the Big Screen being drawn toward the entity with no escape. The ship then begins bending and twisting out of shape as the asteroid had, before it blinks out of existence. Sandra faints.

The object is a black hole (termed "black sun" throughout the episode). Estimates indicate that Alpha has three more days of existence.

Using the eight towers surrounding the Moonbase which create the artificial gravity on Alpha, Professor Bergman has devised an anti-gravity forcefield which - when activated - should negate the gravitational power of the black hole to protect the base. If tests prove successful, it offers a slim hope of the Moonbase holding together once the moon enters the entity. To power the forcefield, the nuclear generators from the Eagles have been removed. Kano (Clifton Jones) is not pleased that Alpha's main computer will have to be deactivated when the anti-gravity screen is switched on to also aid in its powering.

Commander Koenig and Professor Bergman have travelled out to the lunar surface right under one of the gravity towers. The forcefield is activated for the first time, resembling a green dome of light covering the Moonbase. Alan Carter (Nick Tate), flying the one Eagle that has been left intact, moves in and fires a bolt of laser energy right above Koenig and Bergman. The forcefield repels the blast. Everyone celebrates, except for Helena (Barbara Bain), who berates the two Alphans for needlessly risking their lives in such a display. Koenig explains it was to boost morale.

Despite the results, Commander Koenig's faith in the forcefield saving them from the overwhelming force of the black hole is shaky at best, so he announces his plans for the one intact Eagle. It's to be a survival ship for six individuals - three women and three men - to be sent out into space equipped with six months of supplies in the hope that they'll find something in space that'll save their lives.

As the moon is drawn closer to the black hole, the base's interior is getting colder; the entity draining their power. When there are twelve hours left before the moon was previously calculated to be entering the black hole, computer readings indicate that there are actually six hours to go. The moon is being drawn towards it faster as it gets closer. Koenig decides to activate the forcefield immediately in the hope that it will slow the black sun's draw on the moon. Victor enters the base's power room to monitor the levels during the forcefield's operation. After it powers up, a glitch is detected in one of the towers. Bergman opens up a wall panel to check the circuits associated with the gravity towers and ends up getting zapped. The power is drained from his mechanical heart, incapacitating him.

While Victor recovers in the Medical Department, Helena informs him that his mechanical heart saved his life; anyone else would've been electrocuted.

The forcefield is now being repaired with less than four hours before the moon enters the black hole. The survival ship has been prepared and it is now time for the computer to choose the six passengers. It turns out that Alan Carter, Helena, and Sandra are among the crew. Helena refuses to go, but Koenig orders her to. As the ship lifts off, the forcefield is switched on.

With death a real possibility, the Alpha personnel find ways to pass the time; Koenig and Bergman becoming the only occupants of Main Mission. Victor breaks out a 50-year-old bottle of brandy for he and Koenig to indulge in to prepare for the end. "Whatever happens, I just hope it's interesting", Bergman remarking.

As the moon enters the black hole, the Alphans find themselves becoming transparent. Koenig observes his surroundings and realizes with astonishment that the forcefield is holding.

Even though the survival ship is out in space, the Eagle and its occupants have also become transparent. The ship's crew then fades out of existence.

Koenig and Bergman are suddenly surrounded by a shimmering light and no longer feel the biting cold of the Moonbase interior. Within moments they become eons old (it is unknown whether this had occurred to all of the Alphans or if Koenig and Bergman had been singled out). They cease to speak aloud but are able to hear one another's thoughts. The two Alphans are then contacted by a God-like female voice which engages in a brief philosophical conversation with Professor Bergman as to the existence of the universe. Bergman asks why she's never spoken with him before; the unseen being explaining that time moves much slowly for it; only thinking a thought perhaps every thousand years. Victor asks if she's God, but doesn't receive an answer as the voice has faded away.

Koenig and Bergman soon find themselves back to normal within an intact and fully functioning Main Mission. The forcefield has been deactivated (by persons unknown) and sensors indicate that the moon is surrounded by stars with the black hole nowhere in sight. Their current position in space is not known; they could be anywhere in the universe.

As the personnel are returning to normal routines, there's no sign of the survival ship. Suddenly, out of nowhere it shows up on Alpha's screens. Everyone is mystified at its appearance as it had not followed the moon into the black hole. Some of them are seriously starting to suspect a benevolent force has assisted them as the celebratory homecoming ensues.