Andrei Tarkovsky weaves a philosophical meditative narrative into a science fiction movie plot. A mysterious object hit the Earth in a small, unnamed country, creating a dangerous area around it they call "the zone." Only an illegal guide called a "Stalker" can bring people safely in and out of the zone. If you make it to someplace in the center, you get your wish – but it will not be what you asked for -- you will get what you truly want. And there is a price (besides the Stalker's fee).
I noticed around 20 or so years ago that most of Hollywood's "science fiction movies" turned out to be simplistic, loud vehicles for CGI effects, with Manichean plots, pointlessly fast action sequences, and juvenile dialog. But long before this descent into mindless entertainment, Tarkovsky made this strange, slow-paced, beautifully crafted film. He sets the mood then builds suspense by means of dialog and the expressiveness of his actors. He shot in very muted color -- bordering on black and white -- and did not bother with special effects. His characters recite poetry and deliver lengthy monologues, contributing to the dream-like quality. By disconnecting the story from any specific time or place, he made this film timeless.
Be careful what you wish for. You will get it.
The Mechanics Institute has the Criterion Collection DVD of Stalker