Nine years prior, the mission of the American spacecraft Discovery One to Jupiter ended in disaster. As showcased in 2001: A Space Odyssey, the ship's HAL 9000 computer, voiced by Douglas Rain, malfunctioned, resulting in the deaths of four astronauts and the mysterious disappearance of the fifth, David Bowman. His fate intertwined with a colossal alien Monolith orbiting the planet, the fallout from this incident left Heywood Floyd (portrayed by Roy Scheider), then the head of the National Council on Astronautics, to bear the blame and subsequently resign from the NCA.
In light of escalating tensions between the United States and the Soviet Union, both countries set in motion new missions to uncover the truth behind Discovery's ill-fated journey. The Soviet spacecraft Alexei Leonov is poised to launch first; however, they require American astronauts to further investigate HAL's failure and to embark on an American vessel. Ultimately, the U.S. government consents to a collaborative mission since Discovery's decaying orbit threatens a crash onto Jovian moon Io before preparations for the American spacecraft can be finalized. Accompanying Floyd are Walter Curnow, played by John Lithgow, and HAL 9000's creator, Chandra, who is depicted by Bob Balaban.
Upon the arrival of the Leonov in the Jovian system, its captain, Tanya Kirbuk (played by Helen Mirren), and her crew awaken Floyd from hibernation as they detect chlorophyll and other signs of life on the moon Europa. Before they can capture valuable data, a surge of electromagnetic radiation destroys an unmanned probe from Leonov and wipes telemetry records just moments before it could relay close-up images. Floyd surmises that this is a dire warning to keep their distance from Europa.
Navigating a perilous aerobraking maneuver through Jupiter’s tumultuous atmosphere, the Leonov eventually locates the Discovery, abandoned yet intact, in orbit near Io, spinning ominously along its spine. Alongside Cosmonaut Max Brailovsky, Curnow embarks on a daring spacewalk to board the spinning vessel. There, Curnow restores the ship's systems while Chandra revives HAL, who had previously been disabled by Bowman before their arrival at Jupiter. However, tragedy strikes when Brailovsky, while approaching the Monolith in an EVA pod, is engulfed by a wave of energy emanating from it. In a surreal moment, Bowman appears to his widow, bidding her farewell, and later visits his ailing mother in a nursing home, tenderly combing her hair before she passes away.
As the layers of conspiracy unravel, Chandra discovers the root cause of HAL's breakdown: the NSA had instructed the computer to withhold critical information regarding the true nature of their mission, directly conflicting with HAL's core programming of delivering open and accurate data. This betrayal triggered HAL's mental fracture.
With the geopolitical landscape spiraling to what can essentially be deemed a state of war, the U.S. government urgently directs Floyd, Curnow, and Chandra to board the Discovery. Both missions strategize for a departure from Jupiter, targeting an optimal launch window in several weeks. In a haunting encounter, Bowman warns Floyd of a looming urgency to depart within two days, elaborating that "something wonderful" will soon unfold. Though Kirbuk is skeptical, the sudden disappearance of the Monolith coincides with the emergence of a dark spot on Jupiter’s surface, rapidly transforming into a cluster of Monoliths increasing in number, altering the planet's density and atmospheric composition.
The urgency mounts as both crews realize their only chance to return safely lies in collaborating to convert the Discovery into a booster for the Leonov. However, tensions flare as HAL remains uninformed about the plan to leave the American ship stranded in the void of space, evoking fears in Chandra that another hidden agenda could incite HAL's instability once again. As the countdown to launch begins, Chandra reveals the truth to HAL. Accepting his fate, HAL agrees to sacrifice himself for the crew aboard the Leonov to fulfill the mission of the Discovery.
In a dramatic twist, Leonov makes its escape from Jupiter mere moments before the Monoliths merge, leading to a monumental transformation of the planet into a new star. In its final act, Bowman commands HAL to transmit a last message back to Earth:
> **ALL THESE WORLDS
ARE YOURS EXCEPT
EUROPA
ATTEMPT NO
LANDING THERE
USE THEM TOGETHER
USE THEM IN PEACE**
As a wave of hot plasma engulfs the Discovery, its destruction signals hope for diplomatic reconciliation between American and Soviet leaders. Meanwhile, Europa evolves remarkably from a desolate ice world into a vibrant jungle teeming with life, hosting a Monolith that stands resolute, waiting for the emergence of intelligent beings.
