In the prehistoric African veldt, a tribe of hominids are driven away from their water hole by a rival tribe. Later, they awaken to find a featureless alien monolith has appeared before them. Seemingly influenced by the monolith, they discover how to use a bone as a weapon and return to drive their rivals away.
Millions of years later, Dr. Heywood Floyd, Chairman of the United States National Council of Astronautics, travels to Clavius Base, a United States outpost on the Moon, via Space Station 5. At Clavius, Floyd speaks to a meeting of Clavius personnel, and stresses the need for secrecy with respect to their newest discovery. Floyd’s mission is to investigate a recently found artefact buried four million years ago near the crater Tycho. Floyd and others ride in a Moonbus to the artefact, a monolith identical to the one encountered by the ape-men. As they examine the monolith, it is struck by sunlight, upon which it emits a high-powered radio signal.
Eighteen months later, the United States spacecraft Discovery One is bound for Jupiter. On board are mission pilots and scientists Dr. David Bowman and Dr. Frank Poole, along with three other scientists in suspended animation. Most of Discovery’s operations are controlled by “Hal”, a HAL 9000 computer with a human personality. A conversation between Hal and Bowman is interrupted when Hal reports the imminent failure of an antenna control device. The astronauts retrieve it in an extravehicular activity (EVA) pod but find nothing wrong. Hal suggests reinstalling the device and letting it fail so the problem can be verified. Mission Control advises the astronauts that results from their own computer indicate that Hal is in error about the device’s imminent failure; Hal attributes the discrepancy to human error.
Concerned about Hal’s behavior, Bowman and Poole enter an EVA pod to talk without Hal overhearing, and agree to disconnect Hal if he is proven wrong. Hal follows their conversation by lip reading. While Poole is on a space walk outside his pod attempting to replace the antenna unit, Hal takes control of the pod, severs Poole’s oxygen hose, and sets him adrift. Bowman takes another pod to rescue Poole; while he is outside, Hal turns off the life support functions of the crewmen in suspended animation, thereby killing them. When Bowman returns to the ship with Poole’s body, Hal refuses to let him in, stating that the astronauts’ plan to deactivate him jeopardises the mission. Bowman opens the ship’s emergency airlock manually, enters the ship, and proceeds to Hal’s processor core, where he begins disconnecting Hal’s circuits. Hal tries to reassure Bowman, then pleads with him to stop, and then expresses fear. As Bowman deactivates his higher intellectual functions, Hal regresses to his earliest programmed memory, the song “Daisy Bell”, which he sings. When the disconnection is complete, a prerecorded video message plays, revealing that the mission’s objective is to investigate a radio signal sent from the lunar artefact, the monolith, to Jupiter.
At Jupiter, Bowman leaves Discovery One in an EVA pod to investigate another monolith orbiting the planet. The pod is pulled into a vortex of coloured light, and Bowman is carried across vast distances of space, while viewing bizarre cosmological phenomena and strange landscapes of unusual colours. Bowman finds himself in a large neoclassical bedroom. He sees, and then becomes, older versions of himself: first standing in the bedroom, middle-aged and still in his spacesuit, then dressed in leisure attire and eating dinner, and finally as an old man lying on a bed. A monolith appears at the foot of the bed, and as Bowman reaches for it, he is transformed into a foetus enclosed in a transparent orb of light, which floats in space beside the Earth, gazing at it.