American foreign news correspondent Larry Stanford has been working out of Japan for the last few years to the detriment of his marriage. His last assignment before returning to his wife in the United States is an interview with the renowned but reclusive scientist Dr. Robert Suzuki, who lives atop a volcanic mountain.
During the brief interview, Dr. Suzuki amiably discusses his work on evolution caused by sporadic cosmic rays in the atmosphere, and professes that he has discovered a method for producing evolutionary change chemically.
Suzuki serves Larry a drugged libation, causing him to fall into a deep sleep. Announcing to Tara, his voluptuous assistant, that Larry is the perfect candidate for his latest evolutionary experiments, he injects an unknown substance into Larry's shoulder.
Upon waking, Larry is oblivious to the true situation and accepts Suzuki's invitation to spend the next week vacationing with him around Japan. Over the next few days, Suzuki uses Tara as a beguiling distraction while conditioning Larry with mineral baths and copious amounts of alcohol, exacerbating the pain in Larry's shoulder.
Meanwhile, Larry's estranged wife has traveled to Japan to bring him back home with her. But when confronted, Larry refuses to leave his new life of women and carousing. After a few drinks that night Larry examines his painful shoulder to discover that a large eyeball has grown at the spot of Dr. Suzuki's injection.
Becoming aloof and solitary, Larry wanders Tokyo late at night. He murders a woman on the street, a Buddhist monk, and a psychiatrist, while slowly changing form, culminating in his growing a second head. Seeking a cure, Larry climbs the volcano to Dr. Suzuki's laboratory where Suzuki has just informed Tara that Larry has become "an entirely new species" and beyond remedy.
Entering the lab, Larry kills Suzuki and sets the building on fire as Tara flees. Larry splits into two completely separate bodies, bringing himself back to normal. The monstrous second body grabs Tara and falls into the volcano as Larry's wife and the police arrive. Larry, now cured, is taken away by the police, although it remains unclear how much moral or legal responsibility he has for his violent actions. The movie ends as Larry's wife and his friend discuss the good that remains in Larry.
- The Split (International title)
- 双頭の殺人鬼 (Japanese title; Sôtô no Satsujinki - "The Two-Headed Devilish Homicide")