Lee Jung-soo, an automobile salesperson, is driving back toward Seoul when the newly opened Hado Tunnel collapses around him, trapping him inside his car beneath a cruel jumble of concrete and steel. With cellphone reception nearly non-existent, he manages to reach emergency services, and his peril becomes front-page news for a local television crew hungry for a tense rescue story. The rescue chief, Kim Dae-kyung, cuts the reporters off with a blunt reminder: a life is at stake, not a ratings boost. In a tense, practical response, Dae-kyung orders Jung-soo to conserve the phone’s battery, and he imposes a stark rationing plan—two water bottles and a birthday cake meant for Jung-soo’s daughter Su-jin—to stretch time and hope. As the rescue team surveys the darkened tunnel, they realize how close Jung-soo is to being found, yet the tunnel continues to shift and cave in, threatening to bury him and a colleague forever. The clash of danger and determination centers on a simple, stubborn fact: every second matters.
The team identifies a crucial marker—the #3 on a ventilation fan—and Dae-kyung uses this as a coordinate to coordinate two simultaneous efforts: excavating through the tunnel and drilling from above to reach Jung-soo. After days of painstaking work, the tremors from the collapsing tunnel reveal a glimmer of life: Jung-soo hears a dog, a sign that another trapped survivor, Mi-na, is nearby. Mi-na is found impaled by a steel rebar, and Jung-soo reassures her that help is underway, even as the situation grows more dire. Mi-na ultimately dies, and the setback jolts the operation into a painful restart. The team discovers that their earlier tunnel schematics were not properly mapped, forcing Dae-kyung to reevaluate and restart the excavation with renewed urgency. As the days drift by, Jung-soo’s phone dies, severing his last thread to the outside world. Public opinion turns sour on the rescue effort, and a catastrophic accident costs the life of an excavation worker, intensifying the emotional toll on Jung-soo’s family and his wife Se-hyun, who bears the weight of blame from her mother.
Facing mounting pressure, Se-hyun signs an agreement to halt the rescue operation and to restart construction of the Second Hado Tunnel, a plan that would almost certainly seal Jung-soo’s fate. In a quiet moment that tears through the noise of the outside world, she sends a tearful radio message to him, a final message of love and resolve. In a daring, last-ditch maneuver, Dae-kyung descends through the drill hole using a sound detector to pinpoint Jung-soo, while Jung-soo inches toward Mi-na’s car and sounds the horn to guide the rescue. The team detonates the explosives planned for the Second Hado Tunnel, and a wall of vibrations sends fresh rubble raining down, seemingly burying Jung-soo once again. Yet hope lingers in the darkness.
Rescuers push through the debris and finally reach Jung-soo—barely conscious—and Mi-na’s dog, still alive beside him, after thirty-five days trapped underground. In a moment of defiant courage and stubborn humor, Dae-kyung shields the journalists with a profane outburst as Jung-soo is carried away on a stretcher, and Jung-soo voices a quiet, grateful praise for the operation that almost didn’t happen. The victory is bittersweet, and Dae-kyung accepts a desk job, marking the end of a harrowing chapter for him as well. Back above ground, Jung-soo recovers and sits in the passenger seat while Se-hyun drives, the sunlight washing over the tunnel’s mouth and finally letting the two of them breathe freely after their long ordeal.
