The Neanderthal Man

In California's High Sierras, Prof. Clifford Groves' lab window is smashed and the room wrecked, waking his daughter Jan. Groves surveys the damage with fear, sensing a threat beyond a routine burglary, and sends Jan back to bed.

Oakes and Sheriff Andrews document the sabretooth's enormous footprints and begin plaster casting the tracks. Oakes shows the casts to Dr. Harkness in Los Angeles, who confirms the evidence is real and decides to investigate further. The prospect of a dangerous predator draws them deeper into the mystery.

Groves lectures the Naturalist's Club in Los Angeles, arguing Neanderthals were more intelligent due to bigger brains. The club members scoff and demand proof, while Groves retorts with insults. The chairman adjourns, banning him from returning.

Harkness stops at Webb's Café and meets Ruth Marshall, who is stranded on her way to Groves. He gives her a ride to Groves' house, where Jan confirms Groves is away in LA. The encounter links the human drama to the biology mystery.

Oakes and Harkness head out to pursue the sabretooth and manage to locate and kill the creature. Harkness expresses fear that there could be more predators in the area. The successful hunt sharpens their resolve to uncover the truth.

Ruth confronts Groves over their deteriorating relationship, and he ejects her from the house. He then injects himself with the experimental serum and regresses into a Neanderthal. He kills hunter Jim Newcomb and his dog, then returns home and briefly reverts to human form before slipping back toward the Neanderthal.

Back at his lab, Groves writes that the latest regression was the fastest yet and the recovery the slowest. The diary hints at repeated regressions and escalating danger. This note deepens Harkness' concern as unusual events continue to mount.

Buck Hastings is murdered by the Neanderthal Man while Nola Mason is nearby, frightened by the chaos. The creature then abducts Nola from behind a bush, and she remains in terror. The violence accelerates as the mystery tightens its grip on the town.

Harkness and Jan discover Groves' photos showing Celia's regression. They notice a lab cat yowling when a syringe is present, and when injected, the cat turns into a sabretooth and escapes. The bizarre relation between the serum and transformation deepens the peril.

Groves' diary reveals the serum works on cats but not fully on dogs or women, and only partially on men. Jan and Harkness conclude that stopping the Neanderthal Man will require urgent action before the posse closes in. Ruth remains missing, increasing the stakes.

The posse learns the Neanderthal Man has injured Webb and abducted Ruth is missing, prompting a frantic search. They hurry to intercept the creature before Ruth falls into greater danger. Tension mounts as the team narrows the search.

The posse corners the Neanderthal Man in a cave with Ruth. Harkness goes unarmed to coax Ruth to safety, but a sabretooth tiger mauls him as the Neanderthal Man is restrained by the posse's fire. The outcome hinges on human resolve against the monstrous forces they've unleashed.

Back at home, on his deathbed, the Neanderthal Man reverts to Groves for the last time and utters 'Better ... this ... way.' The transformation completes the tragedy and commemorates the danger of unchecked scientific obsession.