The Full Circle (1974)

the exploratory team sent to rheta, a beautiful tropical-looking planet, disappears inexplicably. the other alphani who arrived at the site for research are attacked by a group of primitive men, although the planet does not host any indigenous population. soon the truth emerges: a mysterious fog present on the planet's surface has the effect of reversing the terrestrial on the evolutionary scale, bringing them back to a primordial era. the situation is dramatic: the commander is wounded, sandra is a prisoner and alan risks having to kill his companions to free her.