John Garrity, portrayed by Gerard Butler, is a Scottish structural engineer residing in Atlanta, Georgia, with his estranged wife, Allison and their diabetic son, Nathan. On a pivotal day, John returns home from work with hopes of reconciling with his family as they prepare to host a gathering to watch the passage of the recently discovered interstellar comet named Clarke.
While at a grocery store, John and Nathan are startled by a formation of military cargo jets flying overhead. Shortly after, John receives an eerie automated call that informs him that he and his family have been pre-selected for emergency sheltering. Alarmed by this development, John rushes home just as the comet fragment enters the atmosphere during a live broadcast. However, the situation escalates dramatically when a shockwave from the fragment vaporizes Tampa, Florida, leaving devastation in its wake. Another automated call signals urgent evacuation instructions for reaching Robins Air Force Base, causing panic among the neighbors, who rush to be with their families while the Garrity family hastily prepares to escape.
Before they can leave, a neighbor, Ed, warns them that Clarke is not just a harmless comet, but a collection of large objects poised to bombard Earth over two days, potentially leading to an extinction-level event. Desperate to save her daughter, neighbor Debra pleads to leave her child in the Garrity's care, but John drives off, knowing they cannot take her.
Upon arriving at Robins Air Force Base, John’s family is caught in a traffic jam forcing them to continue on foot, during which Nathan accidentally leaves his insulin behind. They manage to gain entry with a QR code on John's phone but are required to check their belongings. Unfortunately, Nathan's insulin is nowhere to be found. In a frantic attempt to retrieve it, John rushes back to the car while Allison and Nathan wait to board the plane. Just when it seems they may escape, a soldier discovers Nathan's medical condition, leading to their refusal from the evacuation. John, unaware of their fate, is allowed on the plane, but during the chaos of an impending mob trying to breach the base, he has to escape as destruction ensues.
Returning to his car, John discovers a note from Allison indicating they headed to her father's home in Lexington, Kentucky. Meanwhile, Allison and Nathan are picked up by Ralph and Judy Vento, who are headed in the same direction. Ralph, however, has ulterior motives and forcibly ejects Allison, kidnapping Nathan in the process despite Judy's protests.
John finds refuge with a group of survivors and learns of military flights to bunkers in Greenland, but faces chaos when someone challenges his right to the evacuation wristband. As tensions rise, he inadvertently causes the death of someone in self-defense. In a separate plight, Nathan and Allison find themselves at a FEMA camp, where Nathan's honesty about his kidnapping leads to their detention. Fate brings Allison and Nathan back together within the camp, igniting hope amidst the despair.
The next day, John scavenges an abandoned neighborhood for food and water, and while watching news reports of global destruction, he finds a car and makes his way to his father-in-law Dale. Realizing his family is missing, John strives to reunite with Allison and Nathan, which they eventually accomplish after a harrowing journey.
As they make plans to escape to Canada for safety, a sudden alert warns them of impending molten debris. Racing against time, they seek refuge and survive a meteor shower that devastates the region. Eventually, they make it to the airport just in time for an evacuation flight to Greenland, but the escape plane's fate takes a tragic turn when a comet fragment causes a crash landing.
As the Garrity family and other survivors emerge from the wreckage, they brace for impact as Clarke enters the atmosphere—an event that forever changes their lives. Nine months later, as the Greenland base establishes contact with others, John and his family step outside to face a drastically altered world. They are welcomed by the sight of birds soaring overhead—a ray of hope amidst the destruction, as they discover they are not alone, slowly acknowledging their survival.
