John Garrity is a structural engineer living in Atlanta, Georgia, with his estranged wife, Allison, and their diabetic son, Nathan. He returns home to watch the near-earth passing of a recently discovered interstellar comet named Clarke, with his family and neighbors.

While in a store, John receives a strange automated message informing him that he and his family have been selected for emergency sheltering. Confused and concerned, he returns home just as a comet fragment enters the atmosphere on live television. Previously expected to land near Bermuda, the fragment instead strikes Tampa, Florida, vaporizing the city and most of the state. John once again receives a call with instructions to head to Robins Air Force Base for an evacuation flight.

The Garritys then learn from their neighbor Ed that Clarke is actually a giant cluster of objects expected to bombard Earth over the next two days, with the largest piece expected to cause an extinction level event. The family packs up and flees, refusing to take anyone else, as they would be denied boarding.

At Robins Air Force Base, while consolidating their belongings into one bag, the family discovers that Nathan's insulin is in the car. John hurries back for it, while Allison and Nathan are told that they cannot board because of Nathan's diabetes, and are then escorted off the base. Realizing what happened, John leaves the base as a panicked mob breaks in, destroying several evacuation planes when gunfire ignites jet fuel.

Returning to their car, he finds Allison's note that they left for her father's in Lexington, Kentucky. After getting medical supplies from a looted store, Allison and Nathan are picked up by Ralph and Judy Vento. Against Judy's objections, Ralph kidnaps Nathan, hoping to use him and the wristbands to board a flight.

John hitches a ride on a truck with other survivors from the base. A young man named Colin tells him they are going to Osgoode, Ontario, where private planes are flying to Greenland. Another man sees John's wristband and attempts to fight him for it, causing the truck to crash and Colin to be killed. The hostile man is then killed by John before he leaves. Back at the evacuation site, Ralph and a reluctant Judy are arrested after failing to pass as Nathan's parents, and Allison and Nathan reunite shortly afterwards at a FEMA camp across the street.

The following day, John watches the destruction and a countdown to final impact on TV. Stealing a car, he finally reaches his father-in-law, Dale's, and Nathan and Allison arrive shortly after. After the family learn about a complex of underground bunkers near Thule Air Force Base in Greenland where the evacuees are going, John realizes they have just enough time to reach Osgoode. The family then says goodbye to Dale, continuing in his truck.

Hours later, in Upstate New York, they get caught in a traffic jam when a shower of fragments rain down, destroying vehicles and killing numerous people. John manages to drive to an underpass for refuge and they continue to Canada, learning Clarke's largest fragment will make impact somewhere in Western Europe. That night, they arrive at the Osgoode airport and persuade the pilot of the last plane to take them.

The next morning, as they approach Greenland, another fragment strikes off the coast, causing them to crash-land. The survivors flag down a military truck and are taken to Thule, and are quickly ushered inside the bunker complex right as the rest of Clarke enters the atmosphere and collides with Europe, devastating civilization.

Nine months later, the bunkers attempt to make radio contact with other potential survivors as various cities are shown in total ruin including Sydney, Mexico City, Paris and Chicago. The Garritys and others of the shelter exit to see a radically-changed landscape, as Greenland makes contact with other stations around the globe. Each are relieved to hear each other and report that the atmosphere is finally clearing, potentially giving humanity the chance to rebuild.

https://en.wikipedia.org/wiki/Greenland_(film)
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