When research scientist Dr. Gilbert "Gil" McKenna (Clarke) falls unconscious after accidentally being exposed to radiation during an experiment with a new radioactive isotope, he is rushed to a nearby hospital. Attending physician Dr. Stern (Robert Garry) is surprised to find that Gil shows no signs of burns typical for five-minute exposure to radiation and informs Gil's co-workers, lab assistant Ann Russell (Patricia Manning) and scientist Dr. Frederick Buckell (Patrick Whyte), that he will keep the patient under observation for several days.
Later, Gil is taken to a solarium to receive the sun's healing rays. While he naps, he transforms into a reptilian creature, horrifying the other patients. Fleeing from the scene, Gil discovers his new appearance. Stern notifies Ann and Dr. Buckell about the incident, theorizing that the exposure to radiation caused a reversal of evolution, transforming Gil into a prehistoric reptile after exposure to sunlight. Stern suggests that Gil can control his symptoms by staying in the dark and remaining in the hospital, but admits that the patient cannot be held against his will.
Having reverted to normal, a disconsolate Gil notifies Ann of his resignation. Confining himself to his house and only coming out at night, Gil spends his hours drinking and wandering aimlessly around the grounds of his estate. He later drives to a bar where sultry piano player Trudy Osborne (Nan Peterson) is performing.
Buckell soon receives word that noted radiation-poisoning specialist Dr. Jacob Hoffman (Fred La Porta) has agreed to help Gil and plans on arriving in the area within a few days. When radiation poisoning studies offer no leads on solving Gil's own particular symptoms, the distraught scientist contemplates suicide, but soon changes his mind. Instead, Gil returns to the bar where Trudy joins him for a drink and comments that the evening is not over because it is "never late until the sun comes up." Although Gil is disturbed by the comment, his loneliness draws him closer to her. When bar patron George insinuates that he has purchased Trudy's company for the evening, Gil defends her, causing a fight between the two men. After knocking George unconscious, Gil flees with Trudy into the night. Later that evening, after walking the shoreline, they make love, falling asleep in the sand until the morning light awakens Gil. Horrified, Gil flees in his car leaving Trudy stranded on the beach. Arriving at the house, Gil runs in, but not before the transformation occurs.
Ann soon arrives, discovering Gil cowering in the cellar in a state of shock. Believing that he is beyond help, Gil at first refuses to see Dr. Hoffman, but after Ann's tearful pleading, Gil reluctantly agrees. During the examination, Dr. Hoffman orders Gil to remain in the house at all times as a precaution until he can return with help. Feeling guilty for leaving Trudy, Gil returns to the bar but is brutally beaten by George and his gang. Gil regains consciousness the next morning and discovers that Trudy, having felt sorry for him, brought him home to her apartment. George soon arrives and, upon seeing Gil, forces him at gunpoint out into the daylight. Transforming into the creature, Gil murders George in front of the horrified Trudy before fleeing into the hills. Returning to the house, Gil finds Ann, Dr. Hoffman and Buckell waiting there and returns to his normal human state. A disturbed Gil later admits to the murder, with the others assuring him that he acted in self-defense, but when the police arrive with an arrest warrant, the hysterical Gil flees from the grounds in his car and accidentally hits a police officer.
Hiding inside an oil field shack while police comb the area and set up roadblocks, Gil is discovered by young Suzy who offers to fetch him cookies. Hurrying back to her house, Suzy is caught hoarding cookies by her mother and is forced to reveal who they are for. While her mother calls the police, Suzy slips out of the door to return to Gil. Her mother chases after her into the oil field, and police cars soon arrive. Realizing Suzy is endangered by being with him, Gil carries the girl out of the shack into the sunlight where he lets her go. He soon transforms into the creature. In the ensuing police chase, Gil slaughters one of the officers and then climbs the stairs to the top of a tall natural gas tank, where the remaining officer chases after him. As Gil begins to strangle him, the officer shoots Gil in the neck. Mortally wounded, the mutated Gil falls several stories to his death while Buckell, Hoffman and a sobbing Ann watch in dismay.
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