Eddie Jessup (William Hurt) is a psychopathologist undergoing radical research, experimenting with psychedelic substances while immersing himself in a sensory deprivation tank. He hopes the research may help him further understand, or perhaps even cure, conditions like schizophrenia. Eddie starts having increasingly bizarre visions, bursting with bold colors and unthinkable imagery. Those visions fuel his desire to further his research, experimenting with extensive drug use, including ones from indigenous Mexican tribes.
People like me sometimes get eye-rolled for being among the admittedly large number of people overly nostalgic for the movies of the 1970s. But it's easy to explain why the period appeals: The notion that films were much more commonly made for actual grown-ups then is borne out simply by noting that the big Christmas releases for 1971 were as follows: A Clockwork Orange, Dirty Harry, Peckinpah's Straw Dogs, Polanski's Macbeth, and savage satire The Hospital.