Stowaway (2021) Plot Summary & Ending Explained

The crew of the MTS-42 is on a pivotal journey to Mars, comprising commander Marina Barnett (played by Toni Collette), biologist David Kim (portrayed by Daniel Dae Kim), and medical researcher Zoe Levenson (acted by Anna Kendrick). As the mission launches, Marina notices a slight underperformance in the ship's main engines; however, Hyperion's mission control reassures her that there is enough fuel to counteract the issue. Driven by her instincts, she holds off on using the final abort window, aware that this is her third and last mission after two previous ones.

David, focusing on vital research to sustain human life on Mars, views this mission as the first step towards transforming the planet into a second home for humanity. After launching, the vehicle docks with a space station that will be their home during the nine-month journey. The crew takes command of the MTS-Cycler, a station built over successive missions that utilizes its upper stage fuel tank tied to tethers, functioning as a counterweight for inertia-based artificial gravity. David and Zoe, hailing from Harvard and Yale respectively, develop a friendly rivalry as they navigate their tasks.

Soon after takeoff, Marina discovers Michael Adams (played by Shamier Anderson), a stowaway engineer who has been injured and is unconscious behind a panel. Grabbing her medical skills, Zoe quickly stitches him up and manages to save his life with the help of 3D printed parts, though Marina suffers some bruising. Michael, in a state of panic after realizing he had launched twelve hours prior, struggles with the realization that he can't return to his sister. Fortunately, Marina informs him that Hyperion has arranged for a nanny to take care of her.

As the mission unfolds, unexpected challenges arise. Michael's arrival inadvertently compromises critical ship functions when his body damages an essential device that scrubs carbon dioxide from the air, leading to a breach in the life support module. With a grim reality setting in, they realize the sealed-off section leaves only one module an option that is shielded against solar radiation storms.

Taking Michael under his wing, David tasks him with handling data for research tests, but the deteriorating conditions force the crew to resort to emergency lithium hydroxide canisters for CO2 scrubbing, which are limited in sustaining enough oxygen for all four crew members. Burdened with the responsibility of survival, Marina insists that David cultivate his algae experiment immediately, despite the lack of proper supplies, since it is their only chance for long-term oxygen provision.

As time is of the essence, tensions mount among the crew. Although David is hesitant due to missing vital equipment sent ahead to Mars, Marina presses him to activate the algae that can yield sufficient oxygen. With only partial success, the survival stakes climb ever higher; the situation quickly devolves with oxygen quantities dwindling as they push to salvage their mission.

Desperate for solutions, Marina contacts mission control for guidance. However, their only option—a perilous EVA to retrieve oxygen from the launch vehicle—is deemed too risky, leaving Barnett and David contemplating the harsh reality of sacrificing Michael. Their hope flickers as Zoe persuades them to wait, explaining they have a buffer of twenty days and urging to hold on for assistance.

After a heart-wrenching three days of tension, David bravely opens up to Michael, offering him a painless exit. Nearly succumbing to despair, Michael hesitates only to be encouraged by Zoe, who remains steadfast in saving him. With determination, she opts to scale the tethers to reclaim the needed liquid oxygen herself, with David reluctantly agreeing to join her.

Facing deadly conditions during their EVA, they manage to fill two oxygen tanks, essential for survival. Yet, facing a deadly solar flare, they are forced to leave one behind. Upon their return, Zoe accidentally drops the filled tank, wasting their hard-earned efforts and suggesting bleak outcomes as the launch tank begins to leak.

Realization hits hard: one must brave the fray to secure their remaining chance at survival. With nobility, Michael, David, and Zoe all volunteer for this ultimate sacrifice, yet Zoe firmly insists on being the one to retrieve the tank from the launch vehicle. Battling against all odds, she succeeds but ultimately succumbs to radiation—her final moments filled with the awe of Mars glistening amongst the stars, reminding them all of the dream that once united their journey.

In the end, Zoe makes a brave and ultimately sacrificial choice to go outside the ship in space, risking her life to retrieve a crucial oxygen cylinder that had been lost. Her daring act is driven by her desire to save her crew, knowing that the oxygen supply is too limited to sustain all four of them. By risking her life, she manages to bring back the cylinder, which helps prolong the ship’s and the crew’s survival. Unfortunately, her sacrifice isn’t enough to save herself, and she succumbs to the extreme radiation exposure caused by her outside work. The ending underscores the harsh human cost of making impossible choices in life-threatening situations, emphasizing her bravery in the face of imminent death. Ultimately, her selfless act ensures that her fellow crew members can continue their mission and achieve some hope of survival against overwhelming odds. Meanwhile, the other crew members are faced with the reality of their dwindling resources and the impossible dilemma of who will survive and who won't—highlighting the brutal moral and existential questions that space tragedies often evoke. In the final moments, the story leaves us with a powerful reminder of the sacrifices made in extreme circumstances and the tragic cost of prioritizing one life over another.