Dr. Heywood Floyd (Roy Scheider) accompanies a mixed Russian-American crew aboard the Alexei Leonov on a journey to Jupiter to discover what happened to the Discovery mission nine years earlier.
Along the way, the crew discovers evidence of chlorophyll-based life on Jupiter's moon Europa, aerobrakes into Jupiter orbit, rendezvous with the derelict Discovery above the moon Io, boards the craft and reactivates HAL (voice of Douglas Rain).
They learn that the National Security Council informed HAL about the existence of the monolith on the Moon, but instructed HAL not to inform the Discovery crew. This contradiction caused a psychotic breakdown in HAL's programming which Dr. Chandra (Bob Balaban), HAL's creator who is also along for the ride, claims to have corrected.
Both spacecraft leave Io behind and rendezvous with a giant monolith in orbit about Jupiter. Cosmonaut Max (Elya Baskin) pilots a pod to study the monolith, but is killed when the monolith emits an energy burst.
As Soviet-American relations deteriorate on Earth, Dr. David Bowman (Keir Dullea) appears to Dr. Floyd and insists that the two spacecraft depart Jupiter within two days. The only way to do this is to dock the two spacecraft together, use HAL to pilot Discovery as a booster rocket and then detach the Leonov on the trajectory needed to reach Earth. The monolith in Jupiter orbit disappears and this helps Dr. Floyd convince the Russian Commander Tanya Kirbuk (Helen Mirren, in an excellent performance) to implement this plan. The crew decides not to inform HAL of the reason behind the hasty departure.
When the time nears to leave Jupiter orbit, a dark spot composed of millions of monoliths appears on Jupiter and is seemingly consuming the planet. When HAL threatens to abort the launch to study what is going on, Dr. Chandra informs HAL that the crew is in danger and must leave. HAL initiates the escape burn, the Leonov separates and barely escapes as Jupiter collapses and bursts into a second star destroying Discovery and illuminating the solar system.
The creation of a second star in the solar system and a transmission from Dr. David Bowman through HAL informing Earth that a superior alien intelligence was at work leads to peace between the U.S. and the Soviets. As Floyd and the other members of the Leonov crew settle into hibernation for the long voyage home, we see the ice-covered surface of Europa beginning to break-up, some trees and vegetation sprouting and a monolith watching over it all.
While the film is dated based on world events in the late twentieth-century, it is also a perceptive look at long duration-space flight and how a joint Russian-American crew would interact; a situation not unlike what is going on today aboard the International Space Station.
Thanks Evil Ed!