A three-person crew is on a journey bound for Mars when a fourth person is discovered on board. The more the merrier? Not in this case, as there's only enough oxygen on board for three. So either someone needs to hold their breath or an uncomfortable discussion needs to take place.

That's the set-up for "Stowaway," which looks like a sci-fi thriller but unfolds like a straightforward moral play. It's a simple numbers game: someone's gotta go. But how do you measure the value of a life under extreme circumstances?

Zoe (Anna Kendrick), Marina (Toni Collette) and David ("Lost's" Daniel Dae Kim) are the small crew on a two-year journey headed to Mars. Not long into their mission, a member of the launch crew, Michael (Shamier Anderson), is discovered inside one of the ship's panels; turns out he was knocked unconscious during lift-off and was trapped on board. And now it's a little late for him to step off the ship, so he pitches in and helps out where he can.

Except there's not a lot of extra room, extra food or extra oxygen aboard the spacecraft. (Sidebar: The idea that this extra body would not somehow be accounted for back on the ground is more than a bit clunky.) And when David's algae plants don't produce carbon dioxide at the rate they were supposed to, it turns out they have even less oxygen than they expected. So does anyone want to volunteer to abort the mission early?

Co-writer and director Joe Penna (2018's "Arctic") doesn't bog down the narrative with insidious side plots or crew infighting (or by introducing an alien to fight against), he presents the problem and lets the characters work through it. He manages to stage a tense sequence that follows two crew members on an audacious mission to secure extra oxygen. And he maximizes a simple premise by spinning it into an intriguing story about the difficult decisions that need to be made when best laid plans go sideways. Sometimes it's scarier when there's not a monster on board.

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'STOWAWAY'

Grade: B

Not rated: adult language, situations

Running time: 1 hour 56 minutes

Playing: On Netflix