[3] Geologist Patrick Osgood leads a mining team through the network of caves beneath the Alpha complex in search of tiranium, a rare mineral which is an essential component of the Moonbase life-support system.
The search has personal meaning for him as the metal is needed to complete an artificial heart for his dying wife, Michelle.
Semi-conscious, he drifts into the latest in a series of recurring visions he has been having, where Moonbase Alpha is destroyed by a raging fire.
With instruments unable to determine a cause, John Koenig takes Eagle One on a reconnaissance mission to locate the source of the heat.
When the latest artificial heart fails, Osgood denounces these scientific attempts to give life as blasphemy.
After he exits, the doctors debate whether his growing instability is the sign of a breakdown, or, in view of his recent prophetic visions, a more metaphysical influence.
He gives Tony Verdeschi command authority in his absence (as the ship will soon pass out of communications range).
Koenig refuses: with the strain placed on life-support by the present emergency, the scarce mineral is more valuable than a single life.
While visiting the patient, he turns on the charm, downplaying Osgood's irrational behaviour to spare the weakening Michelle any further stress.
While conveyed by stretcher team to Medical, he experiences another vision: Michelle, again lying in the canopy bed on the lunar surface, surrounded by fire.
Helena tests an eleventh model of the Dorfman heart, but it fails to function for any significant length of time.
After passing through and out the other side, Koenig and crew make visual contact with the heat source-—an enormous disc of super-heated plasma and flaming hydrogen.
As he proceeds, the cavern is rocked by a tremendous blast; a super-heated plasma flare spun off from the fire-storm has impacted on the surface.
Verdeschi finally sights the fugitives; when he shouts out the news of the heart, Osgood smashes the security chief's hand-torch with a well-aimed rock and the couple slips away in the darkness.
Sending Michelle back with Maya and the guards, Verdeschi begins digging his friend out from under the rock fall.
With the temperature back to normal and damage repairs underway, Helena and Verdeschi speculate if some sort primal intelligence existed in the fire-storm, it could have somehow made contact with Osgood's unbalanced mind.
Her obvious delight at having saved Michelle's life defuses the situation though, and Koenig returns the smile of the woman he loves.
Series regulars Barbara Bain, Tony Anholt and Zienia Merton carried the majority of the action.
Martin Landau's brief scenes were confined to the Eagle command module; his only interaction with Bain occurred via TV monitor in the epilogue.
This episode featured the return of Zienia Merton after a three-month absence; the actress had left the programme after filming "One Moment of Humanity" because of her reduced involvement (sometimes only a half-day's work) and her new non-contracted 'day-player' status.
[2] Merton recounts that Barbara Bain had telephoned and asked if she would consider meeting with Gerry Anderson and Fred Freiberger to discuss her returning to the series.
Bain's motivation was that she and Martin Landau would be on holiday for the majority of the shooting of an upcoming episode ("The Beta Cloud").
The authors made minor changes in the story, the most obvious being the two Maya transformation sequences feature decidedly extraterrestrial animals: a long-bodied beast with natural armour plating and trap-like mouth would come to Verdeschi's aid in the explosives storeroom and later, a headless, bipedal bat-creature would use its sonar to navigate the Security team through the darkness of the catacombs.
The novel would also give Koenig and Maya more exposure than did the actual shooting script and transformed the character of 'Sahn' into a young Indian man.