A mission, Discovery One, is sent to Saturn. En route, Dr. David Bowman
and Dr. Frank Poole are the only conscious humans aboard; their three
colleagues are in suspended animation, to be awakened near Saturn.
The HAL 9000, an artificially intelligent computer, addressed as "Hal",
maintains the ship. While Poole is receiving a birthday message from
his family on Earth, Hal tells Bowman that the AE-35 communication
unit of the ship is going to malfunction. Poole takes one of the extra-
vehicular pods and swaps the AE-35 unit; but when Bowman conducts
tests on the removed AE-35 unit, he determines that there was never
anything wrong with it. Poole and Bowman become suspicious at Hal's
refusal to admit that his diagnosis was mistaken; Hal then claims that
the replacement AE-35 unit will fail. In communicating with Earth, Poole
and Bowman are directed to disconnect Hal for analysis. These
instructions are interrupted as the signal is broken, and Hal informs
them that the AE-35 unit has malfunctioned.

As Poole is removing the unit he is killed when his pod accelerates into
him, crushing him. Bowman, uncertain of Hal's role therein, decides to
wake the other three astronauts, and therefore quarrels with
Hal, with Hal refusing to obey his orders. Bowman threatens to disconnect him if
his orders are not obeyed, and Hal relents. As Bowman begins to
awaken his colleagues, he hears Hal open both airlocks into space,
releasing the ship's internal atmosphere. From a sealed emergency
shelter, Bowman gains a spacesuit and re-enters the ship, where he
shuts down Hal's consciousness, leaving intact only his autonomic
functions, and manually re-establishes contact with Earth. He then
learns that his mission is to explore Iapetus,[2] in the hope of
contacting the society that buried the monolith on the Moon. Bowman
learns that Hal had begun to feel guilty at keeping the purpose of the
mission from him and Poole, against his stated mission of gathering
information and reporting it fully; and when threatened with
disconnection, he panicked and defended himself out of a belief that
his very existence was at stake, having no concept of sleep.

Bowman spends months on the ship alone, slowly approaching Iapetus.
During his approach, he gradually notices a small black spot on the
surface of Iapetus, and later finds it identical in shape to TMA-1, only
much larger. The scientists on Earth name this monolith "TMA-2", which
Bowman identifies as a double misnomer because it is not in the Tycho
crater and gives off no magnetic anomaly. When Bowman approaches
the monolith, it opens and pulls in Bowman's pod. Before he vanishes,
Mission Control hears him proclaim: "The thing's hollow – it goes on
forever – and – oh my God! – it's full of stars!"