After moving into Cheongun Villa, Dong-won ignores multiple warning signs (uneven floors, rolling marbles, cracked asphalt, trapped elevator doors). Following a drunken celebration with colleagues Seung-hyeon and Eun-joo, the ground yields and the building slips 500 meters straight down into a colossal underground shaft.
Down in the dark, survivors regroup: Dong-won, his son Su-chan, Man-soo, his son Seung-tae, Seung-hyeon (who survived inside a crushed taxi), and intern Eun-joo. They also discover elderly resident Grandmother Oh (suffering from dementia) and her filial son Min-joon trapped in lower wreckage. Despite spirited team efforts, disaster strikes again: shifting debris and sudden collapses claim the lives of Grandmother Oh and Min-joon, shattering the film’s comedic tone and injecting genuine grief.
When torrential rains flood the 500-meter pit into a vertical well, surface rescue attempts fail. Eun-joo and Dong-won realize they must ride the flood upward using the villa complex’s enormous, sealed industrial yellow plastic water storage tank as a buoyancy capsule. In the heroic sacrifice twist, Man-soo seals the hatch from the outside to ensure his son Seung-tae and the others survive. The tank shoots upward like a submerged cork, while Man-soo is torn away by currents. However, upon reaching the surface, Man-soo is revealed to have survived by hooking onto emergency cables and the tank's outer ropes.
Dong-won loses the house he spent eleven years saving for, receiving no magical windfall from insurance. In the epilogue, he and his family live happily in a modest rural camper/suburban home, while Seung-hyeon and Eun-joo marry, illustrating that home is defined not by concrete and square footage, but by the loved ones who survive beside you.