The Hidden Truth About the Virus: Throughout the film, Seoul is locked down under the assumption that the Han River creature carries an unprecedented, fatal virus. However, after Gang-du is captured by American military medical personnel, an American doctor candidly reveals to a Korean translator that there is no virus. The entire biological threat was a fictional narrative engineered to save face and justify extreme military quarantine and chemical deployment. To keep Gang-du silent, doctors perform an invasive lobotomy-like brain operation on him under the guise of "extracting the virus".
The Climax and Hyun-seo's Fate: Subverting standard Hollywood happy endings, when Gang-du finally breaks out of the medical facility and locates the creature's lair near the sewer discharge pipes, Hyun-seo is already dead. In her final moments inside the sewer, she used her body as a protective shield to cover Se-joo, the younger homeless boy. Gang-du pulls his daughter’s lifeless body from the monster’s mouth, discovering that Se-joo is still breathing inside her arms. Driven by immense grief and fury, Gang-du attacks the beast. Nam-il throws Molotov cocktails, Nam-joo fires a flaming arrow into the monster's eye, and a local homeless man douses it in gasoline. Gang-du impales the burning creature through the mouth with an iron pipe, finally killing it.
The Ending: In the quiet, snowy aftermath, Gang-du resides in his small riverfront food kiosk. He has formally adopted Se-joo, serving him warm rice. As news broadcasts play on TV announcing that the virus scare was a total mistake and that Agent Yellow was unnecessary, Gang-du casually kicks the television shut with his foot, completely turning his back on state media and society, sitting in watchful solitude with his rifle to guard his new son.