Mission Commander Marina Barnett, biologist David Kim, and medical researcher Zoe Levenson lift off aboard MTS-42 for a two-year mission to Mars, anchored to the ship’s main hull by a 450-meter tethered upper stage that creates inertia-based artificial gravity. The journey begins with the crew settling into a tightly choreographed routine as they traverse the quiet vastness of space.
Shortly after launch, a startling discovery upends their quiet ascent: launch support engineer Michael Adams, an accidental stowaway, lies unconscious between two modules, entangled with a device that scrubs carbon dioxide from the ship’s air. The moment he shifts, the device is crushed, triggering a chain reaction that compromises their air supply. The crew must improvise, deploying emergency lithium hydroxide canisters to scrub CO2, but the extra burden exceeds what the canisters can handle. The situation grows perilous in real time.
With oxygen thinning, Barnett orders David to accelerate the algae experiment on the ship instead of waiting to set it up at the Martian colony. The surviving algae produce only a fraction of the oxygen needed, offering just enough for a single additional crew member beyond Barnett and Zoe. Facing the grim arithmetic of a four-person life support constraint, they realize they cannot reach Mars with everyone alive unless they find a viable, immediate workaround.
In a tense moment, Barnett reaches out to mission control for a rescue plan, but the only remotely feasible option—a daring EVA to climb the tethers and recover liquid oxygen from the spent upper stage—is deemed too risky to attempt. The weight of potential loss presses on the crew. Barnett and David consider sacrificing Michael, a prospect that weighs heavily, but Zoe urges patience, insisting they wait ten days for a possible alternative.
After three days, David breaks rank to disclose the full gravity of the dilemma to Michael, offering him a painless lethal injection as a possible exit. Michael comes close to ending his life, but Zoe intervenes, urging him to hold on a while longer. She proposes a bold move: she will climb the tethers to retrieve the liquid oxygen herself, while David agrees to join her if needed. David then reveals that the algae’s remaining supply is fading fast, leaving oxygen for only two people. The plan shifts from a rescue to a calculated triage, and Zoe steps into the breach with renewed resolve.
Zoe and David undertake the EVA and manage to jury-rig a system that fills two cylinders—enough for two more people. Yet a high-energy solar flare—a coronal mass ejection—throws a new obstacle into their path: radiation forces them to abandon one of the cylinders. They return to the ship, only to discover a critical equipment fault has caused the remaining cylinder to leak, threatening everyone onboard.
regrouping, they learn that the large tank continues to leak oxygen due to the improvised connection. If one crew member exposes themselves to lethal radiation to retrieve the cylinder left behind on the first attempt, the other three may survive. Marina must stay at the helm, steering the ship, but the choice rests on the others: three volunteers ready to make the ultimate sacrifice.
With Michael untrained and both he and David having families waiting back home, Zoe refuses to gamble with their futures. She volunteers to carry the burden herself. In a harrowing, solitary climb, she secures the needed cylinder and returns it to the ship, but radiation poisons her body. She spends her final moments outside the vessel, gazing at a faint Mars glowing in the distance among the stars, a quiet testament to courage in the face of impossible odds.
