The narrative progresses through an escalating sequence of institutional blunders, personal loss, and administrative abandonment:
After being crushed in the Hado Tunnel, Jung-soo establishes contact with rescue chief Kim Dae-kyung, who teaches him to ration his cake and two bottles of water. While exploring the surrounding rubble, Jung-soo discovers another survivor, Lee Mi-na, who is pinned in her car alongside her pet pug, Ttaeng-yi. Jung-soo shares his water and lets her use his cell phone to speak with her mother, but Mi-na dies from severe internal injuries shortly after, leaving him alone with her dog.
On Day 17, rescue drills finally breach the bedrock, but rescuers discover they have drilled into an empty ventilation shaft. The construction firm used outdated blueprints, deviating from official plans to cut costs. The operation is forced to restart from scratch. Soon after, an accidental detonation kills a rescue technician, causing public sentiment to turn viciously against Jung-soo and his wife, framing the trapped man as an expensive liability holding back the adjacent Second Tunnel project.
Pressured by government authorities, Se-hyun signs a consent form to resume construction blasting. She broadcasts an agonizing final farewell over the radio, believing her husband is gone. Jung-soo hears the transmission on his car radio, realizing the world has abandoned him. Despite orders to stop, Dae-kyung descends into the secondary drill shaft and hears Jung-soo tapping the car horn, which Jung-soo rewired using his car battery. Extracted alive on Day 35, Jung-soo whispers a defiant message that Dae-kyung relays via megaphone to the gathering politicians and cameras: 'Back off, you sons of bitches!' The film concludes months later with Jung-soo and Se-hyun driving toward another highway tunnel; the navigation system chimes, 'Tunnel ahead. Please drive safely,' leaving the couple exchanging an anxious, traumatized look.