The opening text states that before the onset of mankind, other civilizations across the universe had attempted to journey into space. On the planet Somaris, a pilot named Mills (Adam Driver) is with his wife (Nika King), as they watch their daughter Nevine (Chloe Coleman) on the beach. Mills is set to venture on a two-year mission with other passengers in the hope of finding a cure for an illness that Nevine is suffering from. Nevine is trying to whistle but is having trouble doing so, so Mills shows her his own technique.

Sometime into the mission, Mills is commandeering his ship when he comes across an asteroid belt. The ship is hit with chunks of smaller asteroids, causing severe damage to the ship and sending it hurtling toward Earth. Mills crashes in a swamp, with all of the other passengers getting killed after being thrown from the ship. Mills sends out a distress call and begins to survey the environment, discovering that the air is breathable. He then finds one survivor in cryostasis, a young girl (Ariana Greenblatt). As he carries her to safety, he spots a gigantic clawed footprint in the mud. More text appears saying that “65 million years ago, a mysterious visitor arrived on Earth.”

The next morning, Mills continues to check out the environment, when he is attacked by a raptor before he beats it to death. He sees the girl hiding and runs after her, convincing her to go back to the ship with him.

Mills brings the girl to the ship to tend to her wound, learning her name is Koa and that she does not speak English, save for a few words. Mills figures that the pods needed to get off the planet are stuck on a mountain, so he lies to Koa and tells her that they are going to find her family on the mountain. Before they venture off, Koa watches video logs that Mills received from Nevine during his voyage. They are initially light-hearted but she appeared to grow angry and distressed by her father’s absence before the last video Koa sees is Nevine saying that she is very tired. Mills turns the video off and they get moving.

On their trek to the mountain, Koa sees a small dinosaur stuck in mud. Mills reluctantly helps her pull the dino out, but moments later, a pack of predator dinos comes out and fatally maul the little dinosaur, causing Koa to cry. Mills looks up to the sky and sees what looks like a bright object heading toward the planet.

As they continue walking, Mills and Koa begin to slowly bond. He starts to loosen up and act playful with her, and he even carries her on his back as they walk further, and he shows her how to whistle like how he tried to teach Nevine. When it starts raining, they seek shelter in a cave. Mills sets up security monitors, one of which goes off as Koa is sleeping when a parasitic creature goes into her mouth. Mills manages to kill the parasite and save Koa. Soon after, a gigantic predator dinosaur appears and attempts to attack them. Mills shoots it in the face, but it attempts to enter the cave. Mills and Koa drop down a hole to get to safety.

In the hole, Mills tries to find a way out. Koa pulls out a video log from Nevine that she took, which Mills is upset for at first, but he then plays it, although he is too distraught to look at it. He then tries to dig out of the cave before using a small bomb to blow a hole through it. While crawling out of there, Mills and Koa are separated, and Mills finds himself trapped after a slight cave-in. Koa makes the whistling sound she learned to let Mills know she is fine, and she makes it out of the cave, narrowly avoiding pterodactyls and a raptor that she manages to blow up in a log.

After Mills gets out of the cave, he sees the bright object in the sky and uses his device to scan it. His computer tells him that it is an asteroid (THE asteroid) that is set to have a catastrophic impact on Earth in roughly 12 hours. Mills walks until he falls into some quicksand. He calls out for Koa, who manages to grab a long branch and pull him out before he sinks.

The two make it to the top of the mountain, but Koa becomes distraught when she finds the bodies of her parents dead in their cryopods. She gets angry with Mills for lying to her, but he calms her down after trying to explain that he went through the same thing since Nevine died months ago during the mission, and his last memory of her is her being angry with him for not being there for her. He apologizes to Koa and vows to get them out of there.

As they try to start their escape pod, the ship falls further down the mountain and gets disabled. Two T-Rexes approach them and begin to attack. Mills fights them off while Koa stays in the ship, waiting for it to go back online. Mills shoots them both dead, but the cave dinosaur from earlier returns to finish Mills off. He runs toward a geyser to try and get it killed, but Koa arrives to help by stabbing the beast in the eye. The geyser then erupts and melts the cave dino’s face off.

Mills and Koa make it back to the operational ship in time, mere moments before the asteroid makes its impact and decimates the dinos on Earth. The two exit Earth’s atmosphere and head toward an unknown destination.

The credits show Earth redeveloping over time until it reaches its modern civilization.

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Sometime before humans populated Earth, a pilot named Mills from another planet crash-lands on Earth 65 million years earlier into the age of dinosaurs. All of his passengers, save for a girl named Koa, are killed, so Mills and Koa must find an escape pod in the mountains. Mills lost his daughter Nevine while on his two-year mission, so he comes to treat Koa (who also lost her parents) like a surrogate daughter.

The pair must outrun predatory dinosaurs like raptors and T-Rexes while trying to survive. They make it to the ship and fight off the T-Rexes and another large predator just before the infamous asteroid shows up to wipe out the dinos. Mills and Koa escape just in time and fly off to an unknown destination.