Astronaut John (Casey Affleck) awakens from a hibernation cycle aboard the spacecraft Odyssey 1, on a mission to Saturn's moon, Titan. The primary objective is to harvest vast natural resources like methane, which are essential for Earth, before making the long journey back. Since the mission spans several years, the crew must alternate between periods of wakefulness and sleep, typically waking every 90 days for routine maintenance and communication with Earth to ensure their trajectory remains intact.
As John's awareness returns, he reconnects with his fellow crew members, including Captain Franks (Laurence Fishburne)—the mission's leader—and Nash (Tomer Capone), an aeronautics specialist. However, John grapples with the disorienting effects of hibernation, which make him forget critical things, including his girlfriend Zoe's (Emily Beecham) last name. Nash offers reassurance, explaining the severe side effects of the drugs used during hibernation and that eventually, John will piece it all back together.
The crew meticulously checks the ship's systems and informs Earth that they are on target for the most perilous part of their journey: executing a “slingshot” maneuver around Jupiter, which will propel them towards Titan. Just as John recalls Zoe’s last name, the crew returns to slumber.
When they awaken next, John is the first to rise and is suddenly knocked unconscious by a falling ceiling panel. As Captain Franks and Nash investigate, they find evident damage to the hull and some internal components, despite the ship's computers showing no record of any impact. Captain Franks suspects structural strain may have caused the ship to warp.
Nash begins to exhibit signs of paranoia, voicing concerns that the ship’s integrity has been compromised and that they cannot survive the slingshot maneuver. He tries to persuade John to rebel against Captain Franks by forcing their hibernation pods to awaken 12 hours early in order to reprogram the ship for a return to Earth instead of proceeding to Titan. John initially agrees but insists this mutiny should only occur with clear evidence of danger.
They awaken early, but Nash's breakdown intensifies as he expresses doubts about their chances of survival. John remains unconvinced, prioritizing their mission's success. Suddenly, Captain Franks reveals himself, having anticipated their plan and awakening early to prevent it. He successfully leads them through the slingshot maneuver, defying Nash, who continues to spiral further into paranoia and despair.
As their journey progresses, John's own mental state deteriorates, leading him to hallucinate conversations with Zoe and fearful visions of the ship’s decay. When the crew awakens again, Nash appears to have lost significant mental function, failing to recall vital personal details. He brings to John's attention a critical issue with the reactor output that, unless fixed, could prevent them from reaching Titan.
John, recognizing the urgency but still grounded in reality, points out Nash's fragile state. Captain Franks then confirms that the reactor’s issues are intentional sabotage, pulling out a gun and accusing Nash of using John's access code to undermine their mission. In a bid to control the situation, Captain Franks decides that Nash must return to hibernation to thwart further sabotage attempts, escorting both men under the threat of violence.
As John is put under sedation, he witnesses Captain Franks brutally attack Nash, potentially killing him. In the subsequent awake cycle, it’s clear Nash is absent, while Captain Franks grows increasingly aggressive, brandishing his weapon to maintain order. John experiences escalating hallucinations and is led to believe Zoe is attempting to reach out to him. The captain accuses John of losing his grip on reality and shockingly reveals that Zoe was not real; she was a fabricated figment, a psychological test by the company.
Frightened and desperate, John demands to see Nash, suspecting foul play. After a violent confrontation, John manages to overpower Captain Franks. When he uses the ship's computer to locate Nash, it confirms a harrowing truth: John has been entirely alone, sailing through space with only his delusions for company. He is, in fact, Captain John Nash Franks, and everything has been a figment of his imagination.
Amid the chaos of his dissolving reality, John fervently pleads for Zoe's guidance, and she emerges on the radio, revealing the shocking truth: he is not in space at all but trapped in a simulated environment beneath a New Mexico facility, part of an experiment to assess the effects of isolation and hibernation. An earthquake has cut him off from help, exacerbating his unstable mental state.
Zoe implores John to escape via the airlock. With a bloody and unhinged Captain Franks adamantly urging him to stay focused on the mission, John wrestles with the competing narratives in his mind. In a decisive moment, he chooses to plunge through the airlock, entering a real world that is far from the fabricated void of space he believed he was traversing. As he emerges into an underground chamber, clarity washes over him; he recognizes the nature of his hallucinations before he is ultimately pulled into the abyss—the true depths of his mind’s turmoil unraveling in a tragic finality.
