Poor William Hurt. Although he looks young and tasty and naked in Altered States (1980), he has a big problem. After chowing down on mega-hallucinogens and sitting in a sensory deprivation tank for hours on end, he starts de-evolving into a more primitive form of homo sapiens after boning some nice student in one of his mad science classes. Why he does this is never clear — I guess it's just par for the course if you're a biology researcher in the 70s (which is when the movie takes place).
The plot of this Ken Russell joint, such as it is, involves a series of Dr. Jekyll and Mr. Hyde style experiments where Hurt wants to explore alternate psychological states. There is a whole subplot involving religious epiphanies and native people and Hurt trying to deal with being married. Luckily most of the subplots come during his hallucination sequences, and also include him killing a goat with like 100 eyes on it. When he's not hallucinating, he's running around telling everybody incoherent, hippie-ish things about consciousness.
Eventually, he de-evolves into a primordial ooze and realizes that he does want to stay with his wife, even if she's not as hot as that student he was banging when he became homo-erectus in this clip. Oops, I gave away the ending. [Altered States via IMDB]