As the Armstrongs, comprising Dr. Paul Armstrong (Larry Blamire) and his wife Betty (Fay Masterson), set off for an adventure in the mountains during the summer of 1961, they become deeply entwined in a labyrinth of scientific exploration and extraterrestrial mysteries. Driven by an unyielding ambition, Dr. Armstrong seeks to unearth a meteorite believed to hold the rare element atmosphereum, which could dramatically advance his scientific endeavors. Meanwhile, another distinguished scientist in the vicinity, Dr. Roger Fleming (Brian Howe), is engrossed in solving the enigmas hidden within Cadavra Cave, a mysterious locale connected to tales of a "Lost Skeleton".
As darkness descends, the Armstrongs and Dr. Fleming converge at a quaint farmhouse and witness a breathtaking event: a meteorite blazing through the night sky, heralding the start of an extraordinary quest. Yet, their evening soon turns grim when they stumble upon a local farmer, brutally mutilated, a victim of an unseen malevolence lurking nearby.
In the subsequent days, a series of strange occurrences ensue as the Armstrongs and Dr. Fleming discover a mysterious spacecraft, home to two extraterrestrials, Kro-Bar (Andrew Parks) and Lattis (Susan McConnell), who are fighting for survival on this unwelcoming planet. The aliens desperately need atmosphereum to repair their damaged ship, while their mutant pet (Darren Reed) escapes, creating chaos in their wake.
The following morning, Dr. Fleming's deep obsession with Cadavra Cave leads him to the long-missed Skeleton, who compels him to retrieve atmosphereum for his resurrection. Meanwhile, the Armstrongs explore deeper into the woods, finally finding the meteorite that has eluded Dr. Armstrong, situated ominously close to Cadavra Cave, hinting at growing turmoil.
Dr. Fleming's discovery of the Armstrongs' meteorite ignites his treachery, as he schemes to seize it for himself. Simultaneously, Kro-Bar and Lattis employ their clever "transmutatron" to masquerade as locals and infiltrate the Armstrongs' cabin. As the night unfolds, Dr. Fleming creates an alluring ally, Animala (Jennifer Blaire), blending various animal forms to ensure he is welcomed into their cabin, setting the stage for a series of bizarre and unsettling events that will irrevocably change their lives.
As Lattis and Kro-Bar morph into "Turgaso" and "Bammin", they start to understand that Dr. Fleming knows their extraterrestrial origins. An unlikely alliance emerges amid the chaos, with Betty undergoing a psychic probe by the eerie Skeleton, while Dr. Armstrong becomes entranced by Animala's captivating dance.
Unbeknownst to all, Dr. Fleming manipulates the situation, orchestrating a plan to steal the meteorite. As they passively participate in his treacherous game, the Skeleton seizes an opportunity using his mind control powers, freezing Lattis and Kro-Bar just as the prized meteorite falls into Dr. Fleming's hands.
While Dr. Fleming and Animala work to revive the long-lost skeleton with atmosphereum, Betty is confronted by the Mutant's strange advances, ultimately fainting due to the unsettling encounter.
The scene shifts when Dr. Armstrong returns to find his anxious wife waiting by their cabin as the two aliens, now unfrozen, unite under the common purpose of reclaiming their meteorite before it delivers the Skeleton back to life. However, their fragile alliance soon crumbles as they're attacked by the Mutant, initiating a fierce struggle. Despite Dr. Armstrong being injured, Betty’s support brings him comfort, and they retreat to regroup in their cabin.
As the aliens attempt to unravel Dr. Fleming's treachery individually, they find themselves coerced into performing an eerie dance under the Skeleton's control. The sinister atmosphere escalates as the Skeleton exploits his powers to compel Lattis into becoming his unwilling bride, creating discord for Kro-Bar.
In this chaotic environment, Dr. Armstrong devises a strategy to thwart the Skeleton's wicked wedding, enlisting Betty's assistance to draw the Mutant into the fray with promises of havoc. Together, they converge on the ritual's location, launching a surprise attack against Fleming and Animala in a bid to sabotage their defenses.
The conflict intensifies under the Skeleton’s malevolent influence, but just when he intends to finish his defeated counterpart, his hubris leads to his downfall at the hands of the vengeful Mutant. Consumed by anger and mourning over its fate, the Mutant delivers a devastating blow to the Skeleton, casting him over the precipice.
As the dust settles in the wake of this bizarre and violent encounter, the Skeleton lies defeated, his dark energy evaporating along with him. The Mutant, too, succumbs to its injuries, collapsing as life fades from its immense form. With both threats gone, Animala reverts to her original state, transformed back from her assemblage due to the transmutatron's magic.
In the aftermath, the stunned alien-human couples come together to share insights on the vital importance of harmony among different species. United by their trials, they set out to reclaim atmosphereum, leaving behind the chaos crafted by the Skeleton's tyranny and striving for a peaceful future.
