Plot

A three-person crew on a mission to Mars faces an impossible choice when an unplanned passenger jeopardizes the lives of everyone on board.

The crew of a spaceship headed to Mars discovers an accidental stowaway shortly after takeoff. Too far from Earth to turn back and with resources quickly dwindling, the ship's medical researcher emerges as the only dissenting voice against the group consensus that has already decided in favor of a grim outcome.—Theo Rich

After taking off from Earth, the hand-picked, highly trained team of astronauts--Commander Marina Barnett, biologist David Kim, and medical researcher Zoe Levenson--embarks on a two-year scientific mission to distant Planet Mars. However, in their attempt to make the celestial body habitable and find a possible new home for humankind, the crew is not alone. Now, with the flight past the critical point of no return and the fuel and oxygen resources depleting, Marina makes a shocking discovery: the hidden, unconscious, and wounded fourth passenger, Michael Adams. Suddenly, having enough oxygen to breathe becomes the number one priority, and while faced with an agonising ethical dilemma, someone has to die. The question is, who?—Nick Riganas

The astronauts Commander Marina Barnett, biologist David Kim and the medical doctor Zoe Levenson travel to the space station MTS-42 in a two-year mission to Mars. Soon Barnett finds blood on the floor below the life support system and when she removes a panel, an unconscious and wounded black man falls on her and breaks her arm. Soon the crew learns that the man is the support engineer Michael Adams that was trapped in a device that removes CO2 from the air. They befriend Michael that helps them in minor tasks on board to kill the time. When the engineers from the Hyperion advise that the CO2 scrubber is not possible to be repaired, they explain that the station is only capable to support three lives, and Michael must be discarded to save the crew.—Claudio Carvalho, Rio de Janeiro, Brazil

The crew of MTS-42, a two-year mission to Mars, consists of commander Marina Barnett (Toni Collette), biologist David Kim (Daniel Dae Kim), and medical researcher Zoe Levenson (Anna Kendrick). At launch, Marina sees that the main engines are under-performing slightly, but mission control Hyperion says there is enough fuel to compensate. Marina overcomes her instinct and resists using her final abort window. Marina has been on 2 missions previously, and this is her 3rd and final mission. David's research is focused on finding ways to sustain life on a long-term basis on Mars. He believes that his work is the first step towards making Mars a second home to humanity.

The launch vehicle docks with the space station, which will be the crew's home for the 9-month journey to Mars. After taking off from Earth, the upper stage fuel tank of their launch vehicle is held by long tethers to the space station's main hull, acting as a counterweight for inertia-based artificial gravity. The 3-person crew takes control of the space station (called MTS-Cycler), which was itself constructed and assembled over several missions. David is from Harvard and Zoe is from Yale, and the duo have a friendly rivalry going on during the mission.

Shortly after takeoff, Barnett discovers launch support engineer Michael Adams (Shamier Anderso), an accidental stowaway, unconscious behind a panel inside the launch vehicle. Michael is badly injured, but Zoe manages to stitch him up in time and saves his life using 3D printed parts. Michael also fell on Marina's arm and bruised it badly. She gets a cast for 6 weeks.

Michael hyperventilates upon learning that the ship took off 12 hours ago. He needs to get back home as he has a dependent sister, but the ship can't turn around. Marina informs him that Hyperion has hired a full-time nanny for his sister and is covering every expense.

Michael was arming the launch pins when he had a concussion and got trapped behind the panel. Michael gets over the initial shock of waking up in space and offers to help during the mission. The crew begins to bond with Michael, but Barnett soon learns that Michael's body had been entangled with, and inadvertently destroyed, a device which scrubs carbon dioxide from the air on the ship. The device also punched a hole in the hull, making an entire section unusable since it had to be sealed off. The section sealed off is the life support module. Additionally, there is only one module on the ship that is protected and shielded from effects of a solar radiation storm. David takes Michael under his wings for data entry tasks on his research experiments.

The crew are forced to use emergency lithium hydroxide canisters to scrub CO2 from the air. However, the canisters can only sustain enough oxygen for two passengers. Marina soon figures out that the damaged filter cannot be repaired. 4 people need 950 liters per day. Barnett orders David to immediately cultivate his algae experiment on the ship, rather than at the Martian colony as planned. David is reluctant since the equipment to cultivate the algae was sent ahead on an unmanned ship to Mars. but Marina informs him that the CO2 scrubber device is gone. It can't be repaired, and hence algae are the only chance of survival.

David agrees to activate half of his algae supply that produces 550 liters of O2 per day. With no other alternative in sight, Marina asks David to commit his entire batch. Without the proper equipment the algae are not even fractionally as effective as it could be, and risks dying at any time. Only half of the algae survives, providing just enough oxygen for a third crew member. Without another oxygen supply, the crew of four will asphyxiate before reaching Mars. An unmanned resupply mission is not possible as the space station is moving too fast.

Barnett asks mission control for a solution that will save all four passengers, but the only option - an untested EVA to climb the tethers (That risks damaging the solar panels or the tethers could also break) and recover liquid oxygen from the launch vehicle - is deemed too risky. Marina believes that the main engines had to be run longer at launch and so any extra liquid O2 left would be minuscule. They cannot check the liquid O2 levels from the station and need to climb 450 meters to the launch tank to make the check and retrieve the O2. Barnett and David come to terms with sacrificing Michael, but Zoe convinces them to wait ten days for mission control to think of a solution (they have 20 days of buffer).

Marina explains the situation to David and Zoe but asks them to not talk to Michael. But David is not OK with that. Meanwhile Michael feels that he has been accepted by the crew. After three days, David explains the situation to Michael and offers him a painless lethal injection. Michael nearly takes his own life, but Zoe convinces him to hold out for a while longer. Zoe is not willing to give up on Michael. She insists on climbing the tethers to retrieve the liquid oxygen. David reveals that the rest of the algae has died, leaving only enough oxygen for two, and agrees to join her.

Zoe and David perform the EVA and fill two tanks, which would be enough to sustain two more passengers. However, deadly radiation from a high-energy solar flare forces them to leave one of the tanks behind. They make it back to the ship, but Zoe accidentally drops the first tank (which was actually filled), which drifts away into space (so now they have no filled tanks inside the space station). Meanwhile, the large launch tank is slowly leaking oxygen and thus won't have any left for another retrieval.

After regrouping, the crew realizes that one person must retrieve the second tank (which Zoe had to leave attached to the launch tank) so that the other three can all survive. Michael, David, and Zoe all volunteer to make the sacrifice, but Zoe ultimately insists on doing it herself. She manages to fill and return the tank to the ship before succumbing to radiation poisoning. She spends her final moments outside of the ship, gazing at Mars among the stars.