William Hurt (Lost in Space, Hulk) portrays scientist Eddie Jessup who is searching for the meaning of life. As a child and teenager he had religious themed visions. In his personal quest he goes to Mexico, to a tribe of indians who use a mushroom to regress to a more primitive mind-state. He hallucinates and kills and eats a huge monitor lizard. Not quite convinced the substance works, he takes the solution back with him to the States and continues to experiment with it in conjunction with floating in an isolation tank. He becomes obsessed with his experiments, much to the dismay of his wife, Emily (Blair Brown of Fringe). First, he regresses transforms into a four-foot tall primitive human and kills a goat at a zoo, then fast-forwards and changes into an energy being in the finale of the movie, eventually coming to a realization of who he is and what's important to him.
This Ken Russell movie is a great adaptation of Paddy Chayefsky's novel, primarily because the author wrote the screenplay as well. Some of the visual effects/green screen shots are a bit dated, but the makeup effects hold up. Highly recommended.