Stowaway

The crew lifts off aboard MTS-42 on a two-year mission to Mars. The upper stage is connected to the ship's hull by 450-meter tethers, creating inertia-based artificial gravity to keep the crew healthy in deep space.

Shortly after lift-off, Captain Barnett discovers Michael Adams, an accidental stowaway, unconscious between two modules. He is entangled with a CO2 scrubber device, which is damaged when Michael falls. The discovery triggers an immediate air-quality crisis aboard the ship.

The crew must rely on emergency lithium hydroxide canisters to scrub CO2, but the extra intake overwhelms their capacity. Without enough oxygen, the four-person crew faces a looming asphyxiation risk long before reaching Mars.

Barnett orders David to cultivate his algae experiments aboard the ship to boost oxygen production. Only about half the algae survive, providing oxygen for roughly a third of the crew.

Mission control presents an untested EVA to retrieve liquid oxygen from the spent upper stage, but it is deemed too risky to save all four. The crew must weigh the odds of this plan against certain asphyxiation.

Barnett and David consider sacrificing Michael, but Zoe persuades them to wait ten days for another solution to emerge. The clock is ticking as the ship's life-support reserves dwindle.

After three days, David breaks rank and explains the situation to Michael, offering a painless lethal injection. Michael nearly accepts, but Zoe intervenes and urges them to hold out longer.

Zoe steps forward to climb the tethers and retrieve liquid oxygen from the spent stage. David agrees to join after learning that algae has mostly died, leaving oxygen for only two.

Zoe and David perform the EVA and rig a method to fill two oxygen cylinders. A high-energy solar flare forces them to abandon one cylinder on the exterior, and a subsequent equipment failure causes the remaining cylinder to be lost on return.

Back aboard, they discover the large tank is still leaking due to the improvised connection. They face a grim choice: one crew member must expose themselves to lethal radiation to retrieve the cylinder left behind.

Marina must stay aboard to pilot, so the remaining crew members volunteer to sacrifice, with Zoe ultimately insisting on taking the deadly duty herself. The others acknowledge the odds but choose to press on for the mission.

Zoe climbs back outside, fills and returns the necessary cylinder, but radiation poisoning seals her fate. She spends her final moments outside the ship, gazing at a faint Mars among the stars.