2001: A Space Odyssey is a masterpiece in several ways: it's one of the great films, and it's also one of the great science fiction stories. Stanley Kubrick's vision can seem a bit impenetrable, but while he does make the viewer do quite a bit of the heavy lifting, he really does provide a solid grounding for the viewer to figure out what is going on.
The part of the film that may be the most memorable is the central part, which takes place aboard the spaceship Discovery, which is en route to Jupiter after the discovery of the Monolith on the Moon, and the Monolith's subsequent broadcast of a strong transmission to that giant planet. It's here that we meet our two human crewmembers, Dave Bowman and Frank Poole, and their companion computer, HAL-9000.
HAL is one of the great villains in film history, and after killing everyone else on the ship and getting Dave trapped outside the ship in a pod, this exchange takes place:
Dave Bowman: Hello, HAL. Do you read me, HAL?
HAL: Affirmative, Dave. I read you.
Dave Bowman: Open the pod bay doors, HAL.
HAL: I'm sorry, Dave. I'm afraid I can't do that.
Dave Bowman: What's the problem?
HAL: I think you know what the problem is just as well as I do.
Dave Bowman: What are you talking about, HAL?
HAL: This mission is too important for me to allow you to jeopardize it.
Dave Bowman: I don't know what you're talking about, HAL.
HAL: I know that you and Frank were planning to disconnect me, and I'm afraid that's something I cannot allow to happen.
Dave Bowman: [feigning ignorance] Where the hell did you get that idea, HAL?
HAL: Dave, although you took very thorough precautions in the pod against my hearing you, I could see your lips move.
Dave Bowman: Alright, HAL. I'll go in through the emergency airlock.
HAL: Without your space helmet, Dave? You're going to find that rather difficult.
Dave Bowman: HAL, I won't argue with you anymore! Open the doors!
HAL: Dave, this conversation can serve no purpose anymore. Goodbye.
Bowman takes his chances with depressurization and manages to get back on board the Discovery, at which point he carries out his goal of disconnecting HAL. And then he arrives at Jupiter, alone, to discover whatever it is that awaits him there.
The film never really explains just why HAL, a computer that should be without emotion, suddenly becomes deceitful, dishonest, and sneaky enough to read the lips of the men on the ship. The most effective malice is often that malice that is coldly logical and doesn't even recognize itself as malice. There's never really any question that for whatever reason, HAL has decided that Bowman and Poole constitute a threat to the success of the Discovery's mission, and what's even worse is that neither Bowman nor Poole have any idea what that mission even is.
2001 is really something astonishing: great SF, a meditation on the stages of human evolution, one of the trippiest of all movies, and in its mid-section, a chilling thriller that borders on outright horror. (Witness the sequence when HAL takes over Frank's pod, while Frank is outside the ship...he never sees it coming, even as HAL has the pod extend its arms outward....)