A scientist develops a formula which will cause animals to regress to the form of their primitive ancestors, and tries it on himself with disastrous results.
At home in California's High Sierras, Prof. Clifford Groves (Robert Shayne) hears glass breaking and looks up in fear from his book, Neanderthal Man and the Stone Age. He finds his lab window smashed and room wrecked. His adult daughter Jan (Joyce Terry) is awakened by the noise. Groves sends...
At home in California's High Sierras, Prof. Clifford Groves (Robert Shayne) hears glass breaking and looks up in fear from his book, Neanderthal Man and the Stone Age. He finds his lab window smashed and room wrecked. His adult daughter Jan (Joyce Terry) is awakened by the noise. Groves sends her back to bed, telling her that he doesn’t like anyone in his work space, & informing her that he has to go to attend to business.
Meanwhile, Mr. Wheeler (Frank Gerstle) spots a huge tiger while hunting. That night at Webb's Cafe the locals tease him. "Three times the size of a mountain lion and got the tusks the size of an elephant – t'ain't natural", says Danny (Robert Easton). Game Warden George Oakes (Robert Long) comes in. Wheeler leaves and Charlie Webb (Lee Morgan) tells him Wheeler's story. Driving home, a sabretooth tiger jumps onto Oakes's car. He scares it off by honking the car's horn.
Oakes and Sheriff Andy Andrews (Dick Rich) make plaster casts of the giant tiger's footprints. Oakes takes one to Dr. Ross Harkness (Richard Crane) in Los Angeles. Oakes eventually convinces the incredulous Harkness that the cast is real. Harkness says he'll drive up that weekend to investigate.
When Harkness stops at Webb's, waitress Nola Mason (Beverly Garland) introduces him to Ruth Marshall (Doris Merrick), who is on her way to see her fiancé, Groves, but is stranded because her car has broken down. Harkness drives her to Groves's house, where Jan tells them that Groves is in LA speaking before the Naturalist's Club.
Groves lectures the club on his theory that Neanderthal man was more intelligent than "modern man" because Neanderthals had bigger brains. The club members scoff at him and demand proof. Groves responds with insults. The chairman (Marshall Bradford) adjourns the meeting, telling Groves not to come back. Groves angrily says to the empty room that he'll show them proof if that's what they want.
Jan invites Harkness to stay at their house. At breakfast, a grouchy Groves complains about Harkness being there, but Ruth insists that he remain. Oakes arrives and he and Harkness head out to look for the sabretooth. They find it and kill it, but Harkness says he fears there are others.
Back at the lab, Ruth and Groves quarrel about their deteriorating relationship. He throws her out, then injects himself with the serum that he has been using to turn cats into sabretooth tigers. He reverts to the Neanderthal Man. Out in the woods, he kills hunter Jim Newcomb (Robert Bray) and his dog, then returns home and becomes Groves again. He writes in his diary that this most recent regression was the fastest yet and the recovery was the slowest. "I gloried in my strength and ferocity", he writes, noting also that he was overcome by the "hungry urge to kill." Then he spontaneously turns into the Neanderthal Man and runs off. Harkness sneaks into Groves's lab and finds photos that Groves took as he experimentally regressed Celia (Jeanette Quinn), his deaf-mute maid.
Buck Hastings (Eric Colmar) and Nola go on a picnic and he snaps some glamour shots of her. But the Neanderthal Man kills him while Nola is behind a bush changing clothes. As she looks at Buck, dead on the ground, the Neanderthal Man carries her off, kicking and screaming.
Oakes phones Jan and says Buck has been murdered. During the call, Celia sees Nola outside. Harkness carries Nola in. She's hysterical and her clothes are torn. Buck, she says, was killed by something "not human." Then she cries, "He tried to pull me by my hair and then he ... then he ..." and collapses into tears, wailing. Jan calls Webb's, tells Webb what happened and asks him to send for the local MD, Dr. Fairchild (William Fawcett).
Harkness shows Jan and Celia the photos of Celia being regressed to a Neanderthal Woman. Celia signs that she has no memory of it. Harkness then notices that one of the lab cats starts to yowl whenever it sees a syringe. When he injects it (off-camera), it turns into a sabretooth (off-camera) and escapes.
Jan and Harkness read Groves's diary. He has written that the serum works on cats, but not dogs, and not fully on women but completely on men. They set out to find the Neanderthal Man before the State Police and Sheriff's posse can. They stop at Webb's and see that Webb has been injured by the Neanderthal Man. Jan says that Ruth's door has been smashed in and that she's gone. "I reckon he got her, too", says a dazed Webb.
Dr. Fairchild tells Harkness and Jan that the posse has cornered the Neanderthal Man in a cave and that Ruth is with him. Harkness walks to the cave, alone and unarmed, and tells Ruth to let the Neanderthal Man run away. He does, but a sabretooth tiger jumps him. The posse holds off shooting for a while as the Neanderthal Man is being mauled.
Now at home on his deathbed, the Neanderthal Man changes back to Groves one final time and utters his last words: "Better ... this ...way."
