In the 2040s, a Martian research facility known as Tantalus Base has been operational for six months, with its eight-member crew just under twenty hours away from concluding their mission. They are waiting for the spacecraft Aurora, which is en route from Earth to transport them home.
Amidst their research, scientist Marko Petrović uncovers evidence suggesting the existence of life on Mars. Choosing to keep this revelation secret, he concocts a plan for one final Extravehicular Activity (EVA) on the surface. His fellow crew member Richard Harrington drives him in a solar-powered rover to the location where the sample was discovered. Just as Petrović collects soil containing the mysterious biological agent, a sinkhole unexpectedly opens, swallowing him whole.
With time running out, Captain Charles Brunel and crew member Lauren Dalby gear up to venture into the pit to recover Petrović's body. However, Dalby vanishes before help can reach the scene. Brunel enlists Vincent Campbell to descend into the fissure, where he encounters a bacterial organism. To their horror, Dalby and Petrović soon reemerge at the main outpost, transformed into fast-moving, aggressive creatures reminiscent of zombies, exhibiting blackened skin and an absence of their former selves. The situation escalates as Harrington falls victim to one of these creatures, only to rise again as one himself.
As the remaining crew struggles to fend off the zombie horde, Brunel tragically suffers fatal injuries and also reanimates, offering crucial insights into the symptoms afflicting them: intense thirst, memory loss, and aggression. It’s discovered that antibiotics can slow the infection's progression.
In a desperate bid for survival, mission psychologist Robert Irwin takes drastic measures by isolating scientist Kim Aldrich in the hydroponics lab with one of the zombies. Meanwhile, Rebecca Lane is injured during the chaos as they attempt to escape to a rover. Facing a dwindling power supply, the survivors – Campbell, Irwin, and Lane – decide they must reach a second rover left at the fissure site. Under the guise of conducting reconnaissance, Irwin commandeers the vehicle and presses Campbell to abandon Lane, whom he claims is infected, while hiding his own possible infection.
As Campbell and Lane wait for dawn so the solar batteries can recharge, they engage in a deep discussion about the nature of the zombies, with Lane pondering if any vestige of human thought remains within them. While Campbell attempts to provide comfort and falls asleep, he awakens to find Lane has deserted him. Knowing she is likely to transform, Lane cannot dissuade Campbell from pursuing her. In a heart-wrenching moment, she takes her own life by removing her helmet; when she revives, she begs Campbell to end her suffering, which he does, albeit reluctantly.
The peril continues when Campbell and Irwin make their way to the Aurora lander, where the reanimated Aldrich ends up killing the crew onboard. The crew’s ordeal leads to Petrovic and the other zombies becoming desiccated forms. An infected Irwin begins to initiate a launch sequence, taking both himself and Campbell into the unknown of orbit. In a final act of desperation, Campbell dispatches Irwin and ejects his body, along with the contaminated blood, into the void of space.
In a tense message to mission control, Campbell admits he lacks the fuel for a rendezvous and conditions onboard can remain sustainable for several months if a rescue is dispatched. Yet, he warns it may not be wise to send someone, as he fears he might also be infected due to exposure to Irwin's blood. He calculates that it will take approximately 15 minutes for his message to reach them and prepares to await their response, all while he drifts aimlessly in the vastness of space.
