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When Dr. Paul Beecher is summoned to research scientist Dr. Matt Campbell's in-home laboratory, he finds Campbell very ill. Campbell tells Paul that, after many years of experimenting he has found "the answer," hands Paul a small bottle of pills, then dies. Later, Paul delivers his report, indicating that Campbell died of a heart attack, to police detective Buck Donley. Buck tells Paul that Dr. Will Beaumont, an old friend of Paul and head of the nearby university's psychology department, will be coming to review Campbell's work, which he had been funding. At his home Paul, suffering a migraine headache, asks his young daughter Betsy to bring him a pill from a bottle in his jacket. Unknowingly, Betsy gives him one of Campbell's pills. Later, during a consultation with patient Marion Wilkins, Paul feels unwell and asks her to return the next day, then lies down. The next morning, Buck and Carol, Paul's attractive young nurse, meet as they both arrive at Paul's office. Paul has obviously just woken up and is surprised when Buck tells him that there was a report of a prowler in town the night before. Marion's cleaning lady then phones to tell Paul that Marion is very sick. Paul goes immediately to see Marion, but she is utterly terrified of him and after she dies suddenly, Paul finds two puncture wounds in her neck. Paul's own condition worsens and he asks Betsy if perhaps she did not give him the correct pill, but she is uncertain. Paul returns to Campbell's lab where he meets Will and his taciturn assistant, Henry Winston. When Paul asks Will what kind of research Campbell was engaged in, Will states vaguely that it had to do with regressing animals' minds to a primitive state, then reversing and advancing the process. After Will suggests that the work might have applications to human intellect, he adds that Campbell had developed a pill to induce primitive instincts but discovered that it was habit-forming. Henry reports that all of the laboratory animals, except some bats, have died of a virus and burns them in a furnace. Paul and Will leave, while Henry continues to work. Later that night, Paul leaves his home, and the next morning Henry is found dead. Buck asks Paul to examine the body and after they both note the same puncture wounds found on Marion's throat, an autopsy is ordered. When the coroner's doctor suspects that capillary disintegration, the complete destruction of body tissue, may be the cause of death, Buck decides to have Marion's body exhumed. Paul asks Carol to stay with him as he does not feel well and offers to take her to dinner after giving her custody of the pills. At the restaurant, Paul is summoned to the hospital to perform an emergency surgery and after secretly removing the pills from Carol's handbag, asks her to wait for him there. Much to the surgical crew's concern, Paul barely makes it through the procedure, then leaves suddenly. Meanwhile, at the cemetery, Marion's coffin is opened and reveals a corpse devoid of flesh. As the hour is late, Carol leaves the restaurant and while walking home is chased by a man, but safely makes it to her house. However, an elderly lady, walking her dog, is attacked and killed by a deranged Paul. The next day, after Paul learns of the woman's death and realizes that he is regularly regressing to a beast-like state, he arranges for Betsy to stay with an aunt. Paul goes to see Will at Campbell's lab and admits that he killed the woman and suspects he may be responsible for Marion and Henry's deaths as well. When Paul asserts that Campbell's pills have turned him into a killer beast, Will thinks that he has been hallucinating and urges a complete rest, but agrees to stay with him to prevent him taking any more of the pills, which he locks in a drawer. Later, while recording Campbell's notes, Will witnesses Paul's transformation into a beast. Paul kills him and throws his body into the furnace. The next day, after Buck receives the university's lab report confirming capillary disintegration, he wonders if Campbell and Will might have been experimenting on humans and, accompanied by an officer, drives to Campbell's to question Will. Unable to find anyone there, Buck and the officer stumble upon the recorder, which contains a recording of Will being killed by Paul. Meanwhile, as Carol opens the office, a distraught Paul tells her to go home. When Carol spots the syringe of poison with which he intends to end his life, Paul knocks her down then transforms into a beast. After Carol recovers, Paul menaces her, but Buck and the officer arrive and Carol runs out of the house, pursued by Paul. In a nearby glade, Paul catches her and is about to kill her when Buck shoots at him. After Buck catches Paul, a fight ensues in which the officer kills Paul. After his death, Paul reverts to his normal appearance.