Very spoilery so if you don’t know what happens to your descendants, go now.
The astronauts come from Kepler, a planet where the elite of Earth escaped to as Earth was destroyed by pandemics and climate change.
There are repeated flashbacks through the film to astronaut Louise Blake as a child, with her father on Kepler. He wanted to grow trees to repopulate the Earth and told her an area near the weather station Henderson Hub seemed to be showing signs of recovery and regeneration. Her father then returned to Earth in Ulysses 1, as one of the first astronauts to investigate the state of the planet and see if it could sustain life once more. Ulysses 1 disappears though, and no word has been heard since.
On Kepler, fertility is plummeting and by the time Blake herself leaves for Earth in Ulysses 2, no one there is fertile.
After landing on Earth, Blake is taken captive by human survivors, descendants of those abandoned when the elite escaped. Her captain, Tucker, survives the landing but is attacked by the Muds (the dismissive name for those left behind) and later kills himself because of the seriousness of his injuries.
Blake is held alone down an old well, until a commotion there sees young girls and a baby lowered down to hide with her. Above, a marauding force is kidnapping adults and young girls, including Maila, a child who Blake has tried to befriend. Blake follows Maila’s mother, Narvik, as she tries to get her daughter back, with Blake swimming to the boat the kidnappers are escaping on.
They are taken to a large ship, where they are inspected by a neatly dressed man, Paling. He sees Blake’s dogtags and takes her to meet their leader, a man called Gibson.
Gibson remembers her from Kepler and tells Blake that her dad, who had travelled to Earth with him on Ulysses 1, died after siding with the Muds in a rebellion. She is given a room with a shower, and after cleaning herself up discovers her periods have restarted, indicating her fertility has returned.
Gibson introduces her to a school class of children on the ship and to his young adopted son Neil — who shows her a tiny tree he’s growing in a pot, which he got from a man held on the ship supposedly called Christopher Columbus. Blake goes off in search of him and finds it’s her dad. Gibson finds her there — it turns out her father led the Muds rebellion after falling in love with a local woman. Her father tells her no one should have returned to Earth from Kepler.
Blake needs to find her biometer, a device which measures atmosphere and fertility. She wants to transmit the data about fertility on Earth back to Kepler from the Henderson hub, the nearby weather station.
At dinner, the lights go out; Narvik has broken in to rescue her daughter Maila. She is captured, and Gibson tells Blake that Norvak will be executed the next day. The biometer is eventually found in Maila’s doll, which Neil has.
Blake frees all the prisoners, adults and children. Her father tells Blake he’s seen Gibson escaping from the boat. Blake swims to the Henderson Hub and finds Gibson trying to transmit the data back to Kepler. He seizes his adopted son Neil and tells her he will kill the child if she doesn’t drop her gun — and that Neil is her flesh and blood, her brother. Her father is actually Neil’s father.
She puts down the gun and Gibson shoots Neil’s mother, who falls dead into the sea. Gibson sends the data back to Kepler. Blake and Gibson fight and fall into the sea where she strangles him underwater. As Blake falls unconscious in the water, Maila’s mum Norvak appears and drags her out, saving her life.
On their survivors’ boat, Blake’s dad suggests she speak to Neil — she gives him the commemorative moon landing matches pack that her father once gave her, and tells him he’s named after one of the astronauts. She talks to him about how people might now be coming from Kepler, and Neil asks if they will bring trees: “possibly”, she replies.
They reach the cluster of boats where the Muds live. The girls and the baby come out as they see the kidnapped adults and children run towards them through the mist.
I do have some questions, the biggest being that ending. Blake’s dad says she must stop Gibson transmitting the data, but he manages to send it, which means in a few years the humans from Kepler may appear. It’s a depressing situation, highlighting that colonisers can be rebuffed but may simply keep coming. The next cohort will be the descendants of the elite who helped destroy the planet in the first place.