Flight telemetry shows all ship's systems operating normally, and a worried John Koenig orders it flown back to Moonbase Alpha by remote control.
They can find no one aboard...until Helena spies a primitive stone axe and a hairy hand nearly hidden behind a seat in the passenger module.
While the support team reconnoitres the immediate area, Koenig and Helena drive off in a Moon buggy, following a series of trail-markers left by the reconnaissance party.
The team leader reports to Carter that they are approaching an area of mist and hope to catch up with Koenig before dark.
Having left early to scout the area, the astronaut has reached Koenig's Moon buggy by the time a concerned Sandra calls.
Signing off, he follows the markers, but veers from the designated trail to take a short-cut through tall grass—and falls head-first into a camouflaged pit.
Trapped at the bottom of a ten-foot-deep (3 m-deep) pit, he begins to dig hand- and footholds into the soft clay walls.
Despite their weather-beaten faces, rotten teeth and unkempt hair, the chief and his cave-wife are dead ringers for Koenig and Helena.
As they approach the mystery ravine, Carter grabs the moon buggy's control stick from Kano and swerves just before they enter the wall of mist.
The cave chief stands swaying before a shallow grave, half dead and covered with gore, as his mate prepares him for burial.
As Carter prepares to shoot the savage, Bergman forces the astronaut's gun arm upward and the laser blast goes wild.
Meeting up on the other side, they are shocked to find Koenig, sporting an ugly head contusion, sprawled unconscious on the ground just beyond the curtain of mist.
They realise the professor may have been correct: if some unknown force was responsible for regressing Sandos into a caveman, it could have also transformed Koenig into the cave chief.
Ignoring doctor's orders, the Commander flies back down to Retha to save Helena and the other transformed Alphans from Carter's wrath.
She returns to the main cavern in time to see that her posse has recaptured Sandra and are tying her spread-eagled to a frame used to dry animal skins.
Ignorant of her true identity, the astronaut sets his gun to kill, takes aim—and is pole-axed by Koenig's stun blast.
The temporal field present in the mist caused the regression of their bodies (clothes and equipment included) to an earlier stage of evolutionary development; the transformation extended to their minds, giving them a Cro-Magnon level of knowledge and intelligence.
The primitive, percussive compositions would be supplemented with his work from previous Space: 1999 episodes (especially "Another Time, Another Place") and a track from the film Thunderbird 6.
Reportedly, he re-wrote much of the script (penned by American husband-and-wife writing team Jesse Lasky, Jnr and Pat Silver) to realise his desire for this series' first (and only) extensive location shoot.
[4] Though delighted by the large role for her character and the idea of the location work, Zienia Merton recalls the trials of this shooting this episode.
First, director Kellett handed her a leopard skin from which to improvise her own costume; he did not want fashionable caveman 'ready-to-wear'—à la Raquel Welch's 'fur-trimmed Maidenform bra' (in the 1966 film One Million Years B.C.
The weather was abominable, her uncooperative costume repeatedly exposed her bare chest, and a swarm of insects attacked her legs while she lay on the ground after her chase scene.
[6] Production designer Keith Wilson was able to effectively revamp the infamous 'Ice Palace' set from the previous episode "Death's Other Dominion" into the Stone-Age home of the cavemen.
The cavemen were mostly portrayed by the regular Moonbase background extras (including Glenda Allen, Tony Allyn, Sarah Bullen, Andy Dempsey and Christopher Williams).
A student of Lee Strasberg and his Method acting, Barbara Bain went home in her cave-woman make-up to practise her primitive vocalisations.
[7] She recalls how her daughter chose this time to bring her first boyfriend home to meet her parents, only to find her mother's face smeared with dirt while she practised her banshee-like howls.
Writing that the use of "beautiful" location shooting gives the story a "remarkable boost in terms of visuals and excitement", he also applauds the plot twist concerning the Eagle's caveman passenger, the filming of the chase scene involving Benes, and Barry Gray's musical score, which Muir believes "underlines the action brilliantly".
Tubb would make several changes to fit the story into the narrative of his novel: (1) As this story immediately follows 'Collision Course', Retha would appear from behind Atheria after that world's disappearance; (2) Carter and Sandra were romantically involved after her previous (unseen) break-up with Morrow; (3) The temporally-transformative mist was not confined to one area; when Koenig fired into the ground to frighten away whatever was watching him and Helena, mist rose up from the hole blasted in the soil and engulfed the pair.