Before the catastrophic earthquake struck three years ago, scientist Yang Gi-su (Jun Hee Lee) fervently attempted to resurrect his daughter through unconventional experiments. His efforts were abruptly halted by authorities just as the earthquake ravaged Seoul, collapsing buildings in its wake.
However, Gi-su’s experimentation with a mysterious green substance, intended to save his daughter, inadvertently granted him a form of immortality. Surviving the disaster, Gi-su finds refuge in The Apartment, the only structure left standing in the city, now under the control of a special ops unit.
Exploiting his newfound longevity, Gi-su manipulates the residents, promising to adapt humanity to the post-apocalyptic world. Establishing a dictatorial regime, he conducts sinister experiments requiring young subjects, leading to the abduction of Su-na for his failed research.
A rescue team comprising Nam-sam, Ji-wan, and former military member Lee Eun-ho infiltrates The Apartment to save the captive children. Encountering formidable soldiers infused with Gi-su’s reptilian formula, the team engages in intense combat, resorting to decapitation to defeat their foes.
In a climactic showdown, Gi-su’s reign of terror ends as Nam-sam dispatches him, freeing the residents from his scientific tyranny. Symbolically, rain finally falls after months of scorching sun, signaling hope amidst devastation.
While Lee Eun-ho opts to remain and rebuild The Apartment, the rescue team returns home, embracing a semblance of normalcy. As they resume their daily lives, Su-na encourages Nam-sam to pursue romance, symbolizing a newfound sense of joy in their post-apocalyptic world.
With no setup for a sequel, the film concludes on a hopeful note, leaving room for future tales of survival in the ravaged city of Seoul.