Jae-hyeok, Kim Nam-gil, works at the aging Hanbyul Nuclear Power Plant, the town’s namesake energy source and its main employer. He has carried the weight of tragedy since childhood, having lost both his father and brother while working at the plant. Now he lives with his mother, sister-in-law, and his young nephew Min-jae, trying to keep the family afloat. He harbors a quiet dream of leaving the town behind and joining a fishing vessel to bring in money for his family, but he faces constant doubt from those around him, including his childhood friend and fiancée Yeon-joo Kim Joo-hyun, who doubt whether he can really escape the pull of the plant’s life.
Pyeong-seok, Jung Jin-young, is one of the plant’s senior operators who, along with a coalition of concerned workers and anti-nuclear activists, pushes for shutting down the plant due to urgent safety concerns. They pressure the President, Kim Myung-min, to halt operations, but their plea is dismissed by the higher-ups, especially the Prime Minister, who are wary of the economic and political costs of such a move.
Then a powerful earthquake rocks the town, sending the reactor into a dangerous overheated state. The plant’s aging safety systems fail to cool the core, and the situation rapidly escalates from tense to catastrophic. In the ensuing turmoil, officials debate whether to vent radioactive particles to relieve pressure or evacuate the areas closest to the plant. They ultimately choose evacuation for those nearest the facility, but the lack of a clear contingency plan leaves the escape routes gridlocked, turning a public safety decision into a life-or-death race.
Before relief can arrive, the reactor building explodes, claiming many lives among the plant workers. Jae-hyeok pulls his friend Gil-seop, Kim Dae-myung, and others to safety as the area around the plant collapses under the blast and the radiation begins to take its toll. He himself collapses from radiation sickness as he fights to save others.
The town’s residents near the plant are quarantined by the KCDC, and Yeon-joo, having obtained proof of the explosion, tries to alert others. In a troubling turn, a jammer is set up and evacuees are kept inside the evacuation center as authorities seemingly abandon them, with only the junior nurse Mi-sook, Oh Ye-seol, continuing to care for the injured. Yeon-joo and the town’s people push through the confinement and manage to escape, hijacking a bus to carry as many residents as possible to safety.
Firefighters labor to control the situation and cool the reactors, but many of them begin to suffer radiation poisoning themselves. Hang, the President’s ally, sinks into depression over the disaster and orders the firefighters to use seawater to try to decommission the reactors. He and Pyeong-seok uncover a dangerous crack beneath a storage tank in the basement, which threatens to leak fuel rods in a further disaster. When Army engineers refuse to intervene, Hang addresses the nation, calling on plant workers to undertake a perilous operation to seal the cracks.
Jae-hyeok is reluctant at first, but Gil-seop urges him to act for the town’s sake, and he agrees to return with the crew. He calls Yeon-joo before boarding a bus back to the town, determined to do whatever it takes to avert a greater catastrophe. As they work, the crack widens and the team is ordered to retreat. A bold plan emerges: seal the door and simultaneously detonate the damaged tank so that the spent fuel rods fall into the flooded basement, forming a new containment.
The mission requires flawless teamwork and sacrifices, and time is running out. Jae-hyeok, the only one who can operate the necessary explosives and already weakened by radiation, volunteers to stay behind. He braces himself to seal the space and detonate, broadcasting a final message to his family and Yeon-joo via his helmet-mounted camera before the explosion. His selfless act sends the fuel rods into the flooded basement, halting a potentially catastrophic nuclear disaster and saving countless lives, even as he does not survive the mission.
