As the film begins, Hunter (C. J. Thomason) finds himself on an isolated highway, where he encounters Jennifer and her young brother Satchel. While they are halted, they witness shocking mushroom clouds erupting at a distance, decimating several "targets." Tragically, Satchel ends up blinded after staring directly at a nuclear explosion. The bombs produce an electromagnetic pulse that disables Jennifer's car, forcing them to search for an alternative. They discover an old diesel vehicle that remains operational despite the EMP and gather essential supplies from nearby stores.
In their journey, they subsequently meet Elizabeth (Monica Keena), who reveals alarming news of explosions across other major cities within view. Radio reports confirm the catastrophic destruction of prominent East and West Coast cities, leaving them shocked and fearful.
During their quest for sanctuary in a nearby home, Hunter is unexpectedly shot by a frightened child, and with heavy hearts, the group exits, unable to assist the child in distress. They soon arrive at another farmhouse, initially believing it to be deserted. However, their entry is interrupted by Brad (Edward Furlong) and Jonathan (Ross Britz). Brad, armed and anxious, attempts to chase them off, but Hunter manages to disarm him. Jonathan, whose grandparents own the farmhouse, proposes that Hunter's group join four existing survivors: himself, his diabetic uncle Wendell, Brad, and Brad's pregnant wife Angie.
Taking charge, Hunter organizes the relatively small group, instructing them to transfer their supplies into the cellar and fortify it against the impending radioactive fallout. He examines his own gunshot wound and finds a small bottle of antibiotics in the bathroom, a fortunate discovery in a time of despair. Once everything is secured in the cellar, Hunter and Jonathan block the door, resolving to stay safe within for a minimum of a month to dodge the fallout’s worst effects. A Geiger counter reveals surprisingly high radiation levels already present in their surroundings.
As days pass, Jonathan manages to retrieve electronics he had previously stashed within a metal safe — a radio and an MP3 player that remarkably survived the blast. He dedicates his time to repairing the short-wave radio, allowing the group to listen to news of devastation in Europe and other areas caused by nuclear warfare. After successfully fixing the microphone, they start communicating with other survivors holed up in similar bunkers across the States.
A few days later, they are joined by Jonathan's friend Rob (Andre Royo), who, unfortunately, is accidentally shot while trying to gain access to the basement. Having being outside longer, Rob bears radiation burns along with the early indications of radiation poisoning. Though he survives the gunshot, Jonathan is plunged into grief, feeling responsible for his friend's injury, but nevertheless welcomes him into their underground refuge.
Despite their careful measures, the survivors progressively weaken due to the radiation. Wendell, already vulnerable because of his diabetes, is the first to succumb to the circumstances. As they bury Wendell, they are compelled to fend off an assault from desperate survivors, themselves weakened after enduring nearly two weeks of radiation exposure.
Angie experiences severe vomiting caused by radiation poisoning, leading to a tragic miscarriage. Overcome by grief, Brad grows furious as he watches his wife and unborn child perish. In a moment of desperation, he threatens to kill Satchel if Hunter does not save Angie, but Jonathan intervenes, knocking Brad unconscious with a shovel and binding his hands.
After losing contact with the other bunker and hearing a harrowing attack, the group realizes that they are alone, with hope diminishing. Hunter watches as the illness from radiation begins to claim each member of their small band. Tragically, Satchel develops pneumonia, and the group can only look on helplessly as he falls ill and ultimately dies. Hunter is engulfed in guilt, agonizing over having used their limited antibiotics for his own injury. Rob, facing reality, carries Satchel's body outside to bury it, declining to take a gun, asserting that those inside ought to keep it.
In a moment of clarity, Brad seeks forgiveness and is then unbound by Hunter. In the following two days, the others — Elizabeth, Jennifer, and Jonathan, begin to experience severe symptoms, including hair loss and fatigue from the radiation. Hunter and Brad, also beginning to lose their hair, listen intently to sounds from upstairs, reiterating the necessity for the others to conserve their energy in preparation for an impending attack. They come to grips with the likelihood that Rob, who remains outside for two days, is no longer returning.
The anticipated assault unfolds, causing Hunter to realize the untenable position they are in inside the basement. He steps outside following the initial onslaught. Tragically, during the confrontation, Elizabeth is stabbed by one of the intruders and dies in Hunter's grasp. Enraged, Hunter, accompanied by Brad, avenges her death by confronting her assailant, resulting in Brad's demise but ultimately leading to the death of the attacker by Hunter's hand. Battling disillusionment, Hunter returns to the house to discover Jonathan aiming a gun at him; moments later, Jonathan turns it on himself. With the basement now rendered dangerous, Hunter and Jennifer ascend the stairs and curl up together in bed, seeking solace amidst the chaos.
Finally, the film concludes several weeks later, showcasing a grim reality where only Hunter and Jennifer are left alive, gravely ill from radiation exposure. They are seen drinking tainted water from a yard pump while Rob's lifeless body is positioned against a tree, revealing that he managed to bury Satchel but chose not to return to the basement before succumbing to the radiation.
