Dr. Eddie Jessup, a young physiologist, is nominally working on schizophrenic pathology, but reserves his true zeal for experiments with his colleague, Arthur Rosenberg, in isolation-produced hallucination, using himself as a subject. At a party he meets Emily, an anthropology student at Columbia University. The two fall in love. Eddie experiences mystical trances during sex; he reveals to Emily that in his experiments he is seeking to unearth the religious visions that he experienced as a teenager. In the isolation tank, he relives his father’s death while tormented by images of a crucified man with the head of a monstrous goat. The young woman, fascinated with his strangeness, persuades him to marry her. Seven years and two children later, Eddie and Emily—now both professors—are on the verge of splitting up: Emily plans to work for a year in Africa, while Eddie has conceived a trip to Mexico to encounter a tribe that uses psychotropic mushrooms in their rituals. During a dinner party with Emily, Arthur, and a physician friend, Mason Parrish, Eddie reveals that his fanatical search for the “original self” has driven the couple apart. Both Emily and Arthur worry over his increasing thirst for ultimate truth, which he believes rests in humankind’s “physiological memory” of the past. Nonetheless, Eddie departs for Mexico with scientist Eduardo Echeverria. There, a native chieftain assures him that he will experience his original soul during the hallucinatory ritual. The scientist drinks the witch doctor’s brew. Frightening visions assault him: explosions, atavistic dances, and writhing reptiles mingle with alluring images of Emily. When he awakens, disoriented, Eduardo tells him that he has killed a lizard in a fit of drugged violence. Eddie loudly dismisses the ritual as a fraud but is intrigued enough to smuggle some of the hallucinogen back to his home in Boston. Recruiting Arthur and Mason, Eddie begins to experiment with the drug, experiencing ever more exhilarating but disturbing trips. Under the men’s supervision, he takes a large dose before climbing naked into an isolation tank in a makeshift basement laboratory. Arthur and Mason hear his excited description of traveling back to the era of the proto-humans and hunting goats with them. Suddenly, weird barking noises interrupt the monologue. The scientists rush to Eddie’s aid and find him bleeding from the mouth and unable to speak. When Eddie frantically instructs them via pen and paper to x-ray his neck, the scientists find that the physiognomy of his larynx has temporarily changed. The effects of the experiment haunt Eddie even after he seemingly returns to normal; at night, in the bed of one of his students, he finds his arm, chest and feet contorting into ape-like forms. Fascinated, he takes notes on his condition. Emily returns from Kenya with the children. Mason Parrish’s letters about Eddie’s obsession have already worried her; when she finds her husband in a rage over the other scientists’ reluctance to repeat the experiment, she becomes alarmed. Nevertheless, she agrees to review the evidence that Eddie’s genetic and physiological structure regressed during his trip. That night, Eddie returns alone to the isolation tank. He emerges from the tank a shrieking, agile ape-man. A night watchman and a security guard chase the ape-man into a boiler room, where the creature stalks both of them and attacks them with the guard’s club. The ape-man escapes the basement and a pack of feral dogs chases him to the zoo. There, he kills and devours a small sheep. By the time a policeman arrives, the ape-man has reverted to Eddie Jessup who has passed out naked next to the mutilated corpse of the sheep. Emily and Mason bring Eddie home from jail. Eddie’s ecstasy at achieving a primal state terrifies his wife—even more so when Mason reports that a monkey has been sighted in the basement and has nearly killed a man. Later that night, Emily listens to transcripts of the first experiment and becomes convinced that Eddie physically transformed during his trip. After asking Eddie over, she begs him to wait before risking his health in another experiment. But curiosity prevails. A third experiment erupts in noise and blinding light from the isolation tank as Eddie screams, his face deforming. Emily tries to open the tank door, but Mason carries her outside and goes back for Arthur just as an explosion rocks the basement. Only Emily is left conscious. She finds the room a vortex of water, Eddie a gruesome mound of primordial tissue at the center. Finally he reverts to a single pulsing cell. Emily’s touch restores him to human form. The scientists take the unconscious Eddie back to his apartment. As Mason and Arthur bicker over whether to continue the experiments, Emily laments Eddie’s inability to be satisfied with human connection as long as he lusts after absolute truth. But that night, the other men having left, Eddie confesses to his wife that he loves her and wishes he could return to her; if only he had not already gone so far in his pursuit. At that moment, Eddie’s limbs begin to change. His body races through evolutionary history, from ape-man to primordial mass. Emily rushes to him, exhorting him to fight the changes, but his touch turns her into a burning electrical silhouette. Eddie subdues the transformations by force of will and takes hold of Emily. The two at last regain human form in each other’s arms. Man and woman caress each other tenderly.