Will and Samantha, affectionately known as Sam, are a young couple anticipating the arrival of their first child. Will travels from Seattle to Chicago to meet her parents, with the intention of seeking her father, Tom Sutherland, for his blessing to marry Sam. However, tensions rise during dinner, leading to an argument between Tom and Will, prompting Will to leave without disclosing the pregnancy or his marriage plans.
The following day, as Will prepares to return, he receives a distressing call from Sam, who expresses fear over a strange noise and abruptly exclaims, “Something’s wrong,” before the line cuts off. Arriving at the airport, Will discovers that all flights have been canceled due to a seismic incident off the West Coast, which has interrupted communications and electrical systems. In a turn of events, Will revisits Sam's parents, where he and Tom resolve to drive to Seattle together to find Sam.
As the journey begins amidst gridlocked traffic, they are halted by soldiers blocking the interstate. Drawing on his experience as a Marine officer, Tom persuades them to allow passage for their vehicle. That evening, they are abruptly stopped by a police car, only to realize that the driver is a civilian armed with a shotgun. A fierce confrontation ensues, resulting in a serious injury to Tom, who suffers fractured ribs. In a display of strength, Tom subdues the assailant, allowing them to commandeer the police cruiser to tow their damaged vehicle to a nearby reservation.
At the reservation, they encounter Ricki, a mechanic who repairs their car and opts to join them for added support on their perilous journey. As they travel, they stop to gather supplies, but their path is soon obstructed when robbers confront them, stealing their precious gasoline. In a desperate bid to recover their fuel, Will gives chase, ultimately apprehending the culprits. With a dramatic turn of events, Ricki successfully shoots out their tires, resulting in a fiery explosion that engulfs the robbers' vehicle, just in time to reclaim their stolen gasoline. However, shaken by the ordeal, Ricki decides to part ways later that night.
The subsequent day brings new challenges as Tom suffers a collapsed lung, necessitating a needle thoracostomy performed by Will. This harrowing experience forges a bond between them, leading Will to believe he has finally earned Tom's approval. On their journey, they encounter a menacing gang of motorcycle riders, forcing Will and Tom to work collaboratively to escape danger. Sadly, Tom’s condition deteriorates, and he succumbs to his injuries, leaving Will to confront the surreal devastation that lies ahead as he continues to Seattle by foot after burning their disabled vehicle to prevent leaving Tom's body behind.
During his trek, Will is picked up by a family, whom he leads to his estranged father's deserted house in Idaho for rest. He negotiates with them, exchanging their four-wheel drive vehicle for a stay at his father’s well-stocked home until they deem it safe to venture north. As Will approaches Seattle, he is met with the haunting sight of a city in ruins, where he discovers tragic remnants of lives lost, buried beneath layers of ash in their vehicles. Upon reaching Sam's ravaged apartment, Will finds a note from her directing him to a nearby cabin.
At the cabin, Will finds Sam, who is accompanied by Jeremiah, a neighbor from the apartment complex. Soon, the tension escalates when Jeremiah, driven by a delusional paranoia surrounding the disaster, attempts to harm Will, believing he could offer a better life for Sam. In a life-or-death struggle, Will is forced to shoot Jeremiah in self-defense. Shortly thereafter, a volcanic eruption ignites a cataclysmic wave of fire, ash, and destructive force, resulting in a terrifying pyroclastic flow. Will and Sam flee the imminent danger, narrowly escaping the enveloping chaos. With emotions running high, they affirm their love for each other in the face of uncertainty, as they journey north toward an unknown fate, still unclear on the true nature of the calamity they have faced.
