Jessup is conducting a research experiment on sensory deprivation, and the altered states of consciousness that one experiences when the brain is going through this process. Through his research, he is trying to find a link to the illusions that many schizophrenics suffer from, which he believes is linked to religious revelations. During his first experience with the tank, he tested twenty-two students, and only two of these students found the experience to be unpleasant. All reported suffering from hallucinations. Dr. Jessup, who is very intrigued with his work, decides to test the experiment on himself. .
During his first experience in the tank, he reports having a variety of dreams, mostly religious. He also reports experiencing his father's death, which his assistant tells him produced some sobbing. After this first experience, Jessup becomes completely obsessed with his findings, and now is focusing on finding a link to the true self, and the great truth of human life, which he feels he will discover through these tank experiments. He says that while in this altered state, he "blacks out", and cannot see the hallucinations, but can sense them. To intensify the hallucinations, Jessup travels to Mexico, to retrieve a Mexican mushroom. .
The concept of the original self, Jessup believes, is linked to God, and when humans were first created, the primate stage. During his second experience in the tank, the hallucinations become externalized, and an x-ray of him shows one of a primate structure. Afterwards, he begins to have hallucinations of himself, but with gorilla features. No one believes him when he reports his findings, so he continues to do the procedure, but alone. This time during thexperiment, he transforms into a "monkey man", and escapes from the tank. He is very violent, and attacks to guards, a pack of dogs, and kills a sleep in the zoo. He later reports that while in this state, he only had a desire to eat, sleep, and survive, which was the goals of all primates during their time.