The movie begins as a three-person spaceship is launching into space for a two-year mission to Mars. The crew consists of Commander Marina Barnett (Toni Collette), Doctor Zoe Levenson (Anna Kendrick), and Scientist David Kim (Daniel Dae Kim).
When Marina checks on the control panels after the launch, she notices a few drops of blood on the floor. She then discovers that a fourth person, Michael Adams (Shamier Anderson) has been on board the whole time.
While the crew members bond with Michael and they put him to work on the ship, they soon find out that there won't be enough oxygen for four people to survive.
Throughout the rest of Stowaway, the three original team members individually grapple with figuring out how to handle the situation. Marina and David determine that Michael must be the one to die, but Zoe doesn't believe that he should be punished when he didn't choose to be on the vessel in the first place.
How did Michael end up on the spaceship, anyway?
In 'Stowaway,' how exactly did Michael get on the ship?
While the title of the film hinted at the fact that there would be an unexpected extra person on board the MTS-42, it wasn't clear at first whether Michael had intended to be there or not.
When Marina unscrewed the Carbon Dioxide Removal Assembly (CDRA) life support panel in the control room, Michael fell out. His weight then forced him to fall to the ground, which opened a CDRA valve (which is crucial for making the cabin air safe).
He was still unconscious and covered in blood at the time when Marina found him. After Zoe and David tended to his wounds, they sat down with Marina to discuss how it happened.
Michael, an employee for Hyperion (the movie's version of NASA), was working as a "launch support engineer" at the time of take-off. While they didn't initially know if he stowed away on the ship on purpose, he expressed his shock at being in space once he woke up.
He didn't recall how exactly he ended up passing out, but he implied that there had been some sort of accident.
"I was arming the second-stage firing pins and I thought I was, I thought I was buckled in and — I thought I was clipped in," he told the crew members.
The exact details of Michael's accident are not revealed, but it's never again suggested that he tried to purposely join the mission.
Because Michael didn't have nefarious intentions when he stayed on the ship and since he was just trying to do his job when an incident took place, many viewers (and Zoe) took issue with the fact that he would be the one to be sacrificed.
A lot of audience members felt that Hyperion should have protected Michael, since he was also one of their employees. When David's algae venture failed and his mission to Mars became moot, some felt as though an argument could have been made for him to be the one to die.
But, the twist ending of the film took the viewers in an entirely different direction.
Who dies in 'Stowaway?'
Early on in the film, once Michael is part of the equation, the MTS-42 crew members learn that there is only enough stored oxygen for three people to survive. If they do not sacrifice one person, all four will end up dying from asphyxiation.
Zoe has the most sympathy for Michael, as she had learned about his position as a caregiver for his younger sister. She gets the commander to agree to allow Michael to live for another 10 days while they exhaust all of their options.
She later stops Michael from taking the lethal injection that David gave him, and she decides to push an EVA (extravehicular activity) mission to try to fill up oxygen canisters from the launch vehicle.
David agrees to go as well once his algae culture dies and there's only enough oxygen left for two crew members.
During the first journey, David and Zoe must leave prematurely due to an impending solar storm. They manage to get one canister full, but it falls on the way back. They leave a second one at the launch vehicle.
Afterwards, Zoe decides to go back by herself during the storm to retrieve the second canister. She sacrifices herself in favor of the other three, and she dies from radiation poisoning in the final minutes of Stowaway.
Stowaway is available to stream on Netflix now.