Seven days previously, the Moon encountered a massive cloud composed of unknown elements and exhibiting strange characteristics.
Four days previously, Astronaut Tom Graham was dispatched to collect particles for analysis in the hope the causative agent could be found and a treatment devised.
Relief turns to doubt when the staff realises the ship should have exhausted its fuel two days previously, there is no response to hails, and sensors show no life-forms on board.
Tony Verdeschi, Bill Fraser and one of three still-healthy Security guards go to meet the mystery ship as it lands.
After proving resistant to stun-gun fire, the seven-foot-tall, frog-headed, enormously-muscled brute crushes the guard to death with its bare hands.
The security chief has the remaining two guards to report to the travel-tube terminus armed with heavy rocket guns.
The dispassionate voice informs them their life-support system is required for the survival of its race; an agent has been sent to take it—by force, if necessary.
After witnessing the slaughter on the big screen, Verdeschi orders Maya and Sandra Benes to safety in the Medical Centre.
They arrive there and he issues a priority-one order to Computer: all Moonbase doors are to remain locked and will open only on his voice command.
In the overcrowded care unit, an ailing Koenig tries to join his men, but finds the door locked and the commlock panel unresponsive.
The workshop where the spacesuits are inspected and repaired is equipped with a steel-walled vacuum chamber; Verdeschi hopes to decoy the creature inside and evacuate the air, killing it.
The beast has him in its clutches when Maya comes to his aid (having escaped from Medical by transforming into a mouse and crawling through an air-vent into the ventilation ducts).
Before losing consciousness, she recommends using a high-pressure injector gun to administer a massive dose of ionethermyecin, an anaesthetic agent too powerful for use in human beings.
Verdeschi sounds the red-alert klaxon, confusing the invader, then broadcasts over the TV monitors on the communications post nearest to it.
Watching from the corridor through an observation window, Verdeschi sees the creature is affected by neither the toxic atmosphere nor the anaesthetic dose.
The alien voice intrudes, telling them resistance is futile...simply give up the life-support system and die in peace.
As Fraser completes his barricade of heavy-gauge electrical cable and connects it to a mobile generator, Verdeschi and Maya track the unstoppable creature from a safe distance.
It removes the core element from its slot and Moonbase goes dead —- lights dim, air purification and circulation ceases and the temperature begins to drop.
His struggle to return the fragile element to its housing attracts the robot's attention and it moves in for the kill—then seizes up and keels over, inactive.
It intrigued him that the Alphans could not defeat this creature with their considerable technical skills and ingenuity, yet a simple life-form (a bee) could bring about its downfall.
[3] It also took into consideration that series stars Martin Landau and Barbara Bain would be available for only one filming day, as it was scheduled for production during the Landaus' contracted holiday in the south of France.
The extra material included (1) A new 'hook', where the Alphans first encounter the cloud and the viewer witnesses its sinister effect on the Alphans' health (the script's original hook was a 'cold-open' with everyone already ill and Helena narrating her status report; this became the opening of Act One); (2) Additional footage of Fraser beginning work on the electrical barrier; (3) Maya's breakdown and the Weapons Section sequence where they fire at what they hope is the creature's control centre.
[2] Director Robert Lynn encountered actor and former British Weightlifting Champion David Prowse in Harrods when asking for advice on exercise equipment, and hired him for the physically demanding role of the creature.
Immediately after Space: 1999, Prowse was cast in the equally-concealing part of Darth Vader in the Star Wars film series.
The narrative differs somewhat from the broadcast version: to save Verdeschi from the creature, Maya transforms into a two-headed, dragon-bodied, fire-breathing 'Love Beast'.
[7] In the 2003 novel The Forsaken written by John Kenneth Muir, it is stated the events of this story were one of the consequences of the death of the eponymous intelligence depicted in "Space Brain".