- While exploring an Earth-type world several Alphans, including Commander Koenig and Dr. Russel, disappear after sighting a strange fog. Shortly afterwards a race of primitive humans are encountered.
- WARNING: CONTAINS SPOILERS:
The Alphans have come across a very habitable planet that they've named "Retha" (an anagram of Earth). Eagle Six is sent to Retha with seven passengers to conduct feasibility studies of colonizing the planet. However, Moonbase Alpha has mysteriously lost contact with the landing party, who haven't checked in for the past two hours. Commander Koenig orders the ship to be brought back to Alpha by remote control, and the only occupant found aboard is the dead body of what appears to be a Cro Magnon man dressed in animal skins.
Professor Bergman is left in charge of the base while Koenig leads a search party on the planet's jungle-covered surface. Meanwhile, Alan Carter and Sandra Benes are conducting a low altitude aerial search in Eagle Two.
The Commander and Helena Russell are leading the ground search party in a Moonbuggy, attempting to locate the trail of electronic markers left by the Eagle Six crew, while the rest of the party are following Koenig's path on foot. When the Moonbuggy arrives at one of the missing party's markers, Koenig and Helena switch to foot travel. They eventually arrive at a swamp and find footprints of a huge reptilian creature while the shrieking of a smaller animal can be heard hiding among bushes. Koenig adds a fresh marker to the one left by the original landing party, then the pair moves on.
While the rest of the search party arrives at the resting Moonbuggy, continuing to follow the Commander's trail, Koenig and Helena have arrived at a swirling patch of thick mist between two large outcrops of rock. The trail of the Eagle Six crew leads straight into it, so the pair enters it intrepidly.
When night falls, Carter contacts Alpha, reporting that he's lost contact with the landing party. Bergman informs him that they have also. Carter's last message was from the support party following Koenig's trail into an area of mist. Bergman informs Alan that the night will last about two hours, so Carter decides to land the ship and resume the search at daybreak. He "parks" his ship next to the search party's empty Eagle.
After sunrise, Sandra awakens to find that Alan has left the Eagle. She contacts him on her commlock and he explains that he's having a look around. He's just reached the Moonbuggy and he's following the electronic markers. She plans to contact him every thirty minutes to make sure he's OK. Straying off the trail a bit, Alan manages to fall into a large animal trap and loses consciousness on the bottom of the pit. Sandra receives no response the next time she tries to contact him.
Meanwhile, Professor Bergman and Kano have commandeered an Eagle to try and learn the whereabouts of the two missing landing parties.
Carter awakens at the bottom of the pit, and witnesses a trio of stone-aged figures watching him from the ground above. One of them, armed with a spear, jumps into the pit and engages in battle with Carter. Alan is unable to draw his laser, too busy countering the aggressor's movements. Finally, the Spearman picks Carter up off the ground, slamming him back down, causing Alan to again lose consciousness. As the Spearman is about to finish Carter off, he's suddenly distracted by the beeping of Carter's commlock lying on the ground. He picks up the device, accidentally activating audio contact. Sandra's face appears on the small screen, enthralling the caveman. After receiving no vocal response to her monologue, Sandra fearfully deactivates her device. The screen goes blank on Carter's commlock, so the Spearman signals to his comrades, who lower a large branch into the pit for him to climb out.
Eagle Three has just entered the planet's orbit, so Bergman informs Sandra - who has updated the list of the missing to include Carter - that their ETA is ten minutes. While she begins to prepare a meal for the newcomers, Alan again revives at the bottom of the pit. This time, he has a way out, as the Cro Magnons had left the tree branch.
In the Eagle, Sandra opens the ship's exterior door, anxiously awaiting the arrival of Bergman's ship. However, she finds herself standing face-to-face with the Spearman, who recognizes her from the commlock. A chase ensues inside the ship before the Spearman renders her unconscious and carries her home.
Eagle Three lands as Carter hops into the Moonbuggy to race it back to the landing site. Bergman and Kano find Sandra's ship empty, but there are signs of a struggle all around.
Night has fallen again. Inside a series of caves, the Cro Magnon tribe has retired until morning. Fire pits and torches provide some light inside the caverns. The Spearman enters with an unconscious Sandra over his shoulders, which draws the attention of the group. A fight breaks out between the Spearman and the leader of the tribe, as the spectators react with a series of grunts. This time, the Spearman is not the victor, having lost possession of Sandra to the Tribe Chief, whose mate responds with violent jealousy. After the defeated Spearman scurries away, the now-conscious Sandra retreats to a remote recess in the cavern, terrified.
Out in the dark wilderness, Bergman, Kano, and Carter are stationed silently around a campfire, having just finished a meal. Alan barely ate, worried about Sandra, and agitated that he'll have to wait for sunrise before searching for her.
Meanwhile, the Cave Chief and his mate have approached Sandra with a flaming torch to get a better look at her, and in the improved lighting, Sandra suddenly realizes that the pair bears an uncanny resemblance to Commander Koenig and Helena Russell. She lets out a loud shriek, then shakenly tries to communicate with the pair. She realizes they're unable to speak. Later on, when the rest of the cave people are asleep, the Spearman approaches Sandra with an animal skin for her to put on, determined for her to become his mate. Sandra's unresponsive, so the impatient Spearman rips her uniform open, since she won't remove it herself. She screams, awakening the surrounding sleepers, causing the Cave Chief to resume battle with the Spearman. This round ends with the Chief bodily throwing the Spearman into a fire pit. The burning caveman rolls around on the ground, extinguishing his burning animal garb. Now it's the Chief who forces Sandra to don the animal skin. When he notices her medical monitor (which resembles a wristwatch that all Alphans wear), he reaches for it, Sandra swiftly picks up a heavy rock with both hands, repeatedly slamming it into the Chief's head. As he lay unconscious with blood on his face, which also spattered onto her discarded uniform, Sandra finally has the opportunity to search for an exit. Sensing a lack of activity where Sandra had been cowering, the Chief's mate investigates, letting out a shrill scream. She then frantically waves Sandra's blood-spattered uniform in front of the others, her way of explaining what happened. Sandra escapes outside, making a bare-footed break for it in terror. She runs blindly through the dark forest, as some of the tribe has emerged from the caves in pursuit. When she's at the point of exhaustion, she collapses behind a large tree, sobbing.
At daybreak, the Alphan trio hops into the Moonbuggy to search for their people. Following the markers, Kano - who's driving - is about to plow the vehicle into the cloud of mist, causing Carter to yell for him to stop. He explains that everyone who has entered the mist have never been heard from again. Bergman suggests they abandon the Moonbuggy and make their way around the area, avoiding the mist.
The cave people are all chanting, swaying in unison, performing a primitive funeral service, preparing to bury their dying Chief, apparently while he's still breathing. They're disrupted by the voices of the approaching Alphans who have found the cave entrance located near the backside of the mist, causing the tribe to flee, abandoning the dying Chief. Victor observes the horrific condition of the lone caveman, then moves in closer when recognition starts to set in. Even though he's dying, the Chief starts to attack, followed by Victor preventing Alan from destroying him with a laser. After the Chief runs out of the cave, Kano picks up Sandra's blood-spattered uniform off the ground, which sets Carter off. He grabs a lit torch with one hand while holding his laser with the other, calling out Sandra's name, disappearing into the deeper caverns. Victor informs Kano that he thinks the injured caveman was the Commander, so the two race outside to find out, back the way they came.
Meanwhile, the tribe scouts have located Sandra, who had fallen asleep behind the tree she'd taken refuge under.
The two Alphans find Koenig unconscious by the moonbuggy, right in front of the cloud of mist that he'd apparently just emerged from, wearing his usual uniform, but suffering a huge contusion on his forehead. As they prepare to rush the Commander back to Alpha.
After Koenig revives on Moonbase, Dr. Mathias reports his findings on the autopsy of the body found on Eagle Six. He appears to be a Cro Magnon, but there were caps on his teeth. Further tests revealed that he was Sandor, the pilot of Eagle Six. Koenig realizes that the other cave people could be Alphans. Victor becomes alarmed, explaining about Carter vengefully searching for Sandra with a laser. Despite his condition, Koenig jumps out of bed and heads for an Eagle. Victor and Kano follow.
In the meantime, the Cave Chief's grave will soon contain a different body. Sandra's been bound to a primitively-made vertical rack used to dry fresh animal skins, surrounded by the tribe who will witness her public execution by the vengeful Cave Helena.
While Koenig's ship lands on the planet, Carter has finally located Sandra. On a landing overlooking the bloodthirsty tribe, he observes the Chief's mate sadistically hovering a sharp piece of wood over Sandra's face and body, preparing to do her in. The fear-stricken Alphan notices Carter in the shadows, who motions for her not to give him away. Alan has his laser aimed right at the Cave Helena as the three Alphans suddenly pile into the cave. Koenig notices Carter and stuns him to prevent anyone from getting hurt. They manage to take Sandra outside, where they use tactics to force the attacking cavemen back into the mist. They revert back to themselves.
Back on Alpha, the landing party members are examined, but fortunately no side-effects are found.
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