Description
Dr. Donald Blake (Arthur Franz), a science professor at Dunsford University, receives delivery of a coelacanth. A student, Jimmy (Troy Donahue), asks Blake if the fish is really a million years old. Blake replies, "It's the species that's old. No change in millions of years. See, the coelacanth is a living fossil, immune to the forces of evolution". Blake lectures his students about evolution and devolution, telling them that man is the only creature that can decide whether to move forwards or backwards, and that "unless we learn to control the instincts we've inherited from our ape-like ancestors, the race is doomed".
Inside the lab, Blake scratches himself on the teeth of the partially-thawed coelacanth, accidentally sticking his bloody hand into the water-filled container which held the fish. Molly Riordan (Helen Westcott), assistant to Dr. Cole Oliver (Whit Bissell), is with Blake and offers him a ride home. When they get to Molly's car, Blake says he doesn't feel well and passes out.
At Blake's home, Molly is attacked by person or persons unknown. Madeline Howard (Joanna Moore), Blake's fiancée and daughter of Dr. Gilbert Howard (Alexander Lockwood), president of the university, arrives and finds the home in shambles and Blake moaning on the ground. Madeline calls the police after seeing Molly hanging by her hair in a tree, her eyes wide, dead.
Detective Lt. Mike Stevens (Judson Pratt) and Detective Sgt. Eddie Daniels (Ross Elliott) find a huge "deformed" hand print on a window and Blake's tie clasp in Molly's dead hand. They take Blake downtown when he admits that he can't remember anything after getting into Molly's car.
Stevens releases Blake after concluding that someone is holding a grudge and trying to implicate Blake in Molly's murder. He assigns Daniels as Blake's bodyguard and tells Blake that Molly's autopsy showed she died of fright.
In his lab, Blake shoos away a dragonfly that lands on the coelacanth. The dragonfly later returns, now grown to two feet in length. Blake and Jimmy try to catch the giant insect with a net when, again, it lands on the coelacanth. Blake stabs the dragonfly. When he examines its body, he doesn't notice that its blood has dripped into his pipe. Lighting up and taking a few puffs, he immediately feels ill. As the dragonfly shrinks back to its normal size, a large, hairy hand reaches out and squashes the insect. Then Blake's lab is trashed and Jimmy's visiting girlfriend is killed. The police find huge footprints near her body and conclude that the footprints and the hand prints are from the same source.
Blake learns that the coelacanth has blood plasma preserved by gamma rays. If it gets into the bloodstream of an animal or person, it causes them to temporarily revert to a more primitive state. He realizes that he might have received a dose of the irradiated plasma. If so, then Blake has been reverting to a throwback caveman with large hands, feet, dark skin, heavy body hair, and prominent brow ridges.
He decides to take a few days off at Dr. Howard's remote cabin. Blake isn't there for rest and relaxation: He plans to learn whether he is the beast. Blake rigs the cabin with cameras on trip wires to record whatever happens and injects himself with coelacanth plasma. His caveman self wrecks the room, trips the camera's wires, and is photographed. He grabs an axe and leaves.
Madeline, driving to the cabin, runs off the road and crashes when the caveman appears in her headlights. A local forest ranger (Richard H. Cutting) arrives and calls the Dunsford police for help. The caveman carries the unconscious Madeline into the forest, with the ranger in pursuit.
Madeline comes to and struggles with the beast. When she breaks free, the forest ranger shoots the caveman, but he throws his axe, killing the ranger. Madeline runs to the cabin. The caveman collapses. Blake, once again himself, returns to the cabin and develops a photo, showing it to Madeline, who does not understand and asks why the beast is wearing Blake's clothes.
Lt. Stevens, Detective Sgt. Powell (Phil Harvey), and Dr. Howard arrive at the cabin. Blake tells them that he not only knows who the murderer is, but where to find him. Out in the woods, he explains to Howard what his experiment proved and injects himself with coelacanth plasma. Again transformed into the caveman, he chases Howard, forcing the two detectives to shoot him. As the beast lies dying, he slowly transforms back into Blake.
