Io is a science fiction post-apocalyptic film. A deadly gas is wiping out humanity (and actually non-plant life). Luckily humanity built an outpost on Io, one of Jupiter's moons, and survivors have been fleeing there.

Sam (Margaret Qualley) is a researcher working with her father looking for a cure. She lives on a house on a hill (the gas clings to the lowlands) and travels to the valley collecting samples. But the last shuttle to Io is leaving soon and her boyfriend urges her to escape Earth.

One day Micah (Anthony Mackie) arrives in a fairly sophisticated balloon. He's here to see Sam's father because it turns out dad's broadcast urging people to stay on Earth indirectly cause Micah's wife's death. After a couple of days of waiting Micah discovers that Sam's father is never coming and she admits he died a year earlier.

Micah forces Sam to come with him and make for the last shuttle. To do that they need hydrogen, to be found in the deadly abandoned city in the valley. Can the two of them make it to safety?

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SPOILERS

Eventually Sam decides to stay and to make sure she takes off her oxygen mask and breathes the gas. In an epilogue we see Sam with her son (with Micah) sending a message to Micah out in Io.

I had to read Wikipedia to see that I missed the plot turn. Sam was born after the gas and there was a clue that bugs born after the gas were immune. So a new generation of humanity can live on Earth again.