In short: Earth sends a spaceship to investigate what turns out to be the catalyst for life on Earth, and possibly the whole galaxy. Along the way, the ship’s computer flips out and tries to kill everyone. The survivor gets to the catalyst, and ascends to a higher state of being. The whole thing is wrapped in symbolism, metaphor, hard science, and lots and lots of space tape.
The books are a heckuva lot better.
@Captain_Fantasy
My dad tells me that his high school class went on a field trip to see it, and that (judging by the screams when they got to the end) the sorts of kids who sit in the back row had been enjoying the film whilst on acid.