The plot turns when zero-visibility fog causes a chain-reaction pileup on the Incheon Grand Bridge. A secret military truck transporting Project Silence bio-weapons is struck, freeing the Echo canines. Dr. Yang attempts to activate their control chips via audio transmitters, but the alpha dog, E-9, breaks free from her device. It is revealed that scientists tortured E-9 and killed her puppies in front of her to maximize her aggression, before cloning the rest of the pack from her enraged DNA. Driven by trauma, E-9 attacks a rescue helicopter, causing it to crash into the bridge's main cable towers.
As the bridge collapses, political candidate Jung Hyun-baek learns of the leak. Rather than deploying rescue teams, he orders special forces to seal both ends of the bridge, lock down communications, and abandon the survivors to suppress the scandal before election day.
Realizing his boss has betrayed him, Cha Jung-won turns against the administration. He works with Jo Park, Dr. Yang, and other survivors, using vehicle lights, noise distractions, and makeshift fire-torches to fight through the foggy bridge. During the final escape, E-9 corners Jung-won and Kyung-min. However, when E-9 discovers her last surviving pup in danger near the collapsing bridge edge, she leaps to save her offspring, sparing Jung-won and Kyung-min.
Jung-won escapes the bridge with his daughter and the survivors. Instead of protecting his career, he releases recorded audio files and evidence of Jung Hyun-baek’s cover-up order to the press, destroying the politician's campaign. The film ends with Jung-won making peace with Kyung-min, choosing to remain in South Korea as a present, loving father. In a final shot, E-9 is seen swimming ashore with her surviving puppy, free from human control.