Yong-nam, Jo Jung-suk, once stood among the best rock climbers in his college days, but today he’s a jobless adult living with his parents, scrambling to keep up appearances as his mother’s 70th birthday approaches. His older sister Jung-hyun, Ji-yeong Kim, pushes him to straighten up, while their mother Hyun-Ok, Du-shim Ko, hopes for a celebration that feels earned.
For the party, Yong-nam insists they celebrate at the Cloud Garden, secretly hoping to see his old crush and fellow ex-climber Eui-joo, Yoona. When he meets her, he pretends that he now has a successful career, a lie that catches her by surprise and seeds a strange tension between them.
Outside, a terrorist parks a truck near the Cloud Garden and releases a toxic white gas, sending the city into a panic. Eui-joo learns from a close friend, via phone, about Yong-nam’s fabrication of his life, which makes her more curious about the truth behind his bravado.
As the gas closes in, Yong-nam’s family rushes back toward the Cloud Garden, but Jung-hyun stumbles and is quickly overwhelmed by the toxic cloud. To reach safety together, they sprint to the roof, only to find the door barred. With quick thinking, Yong-nam shatters a window and uses a makeshift harness to scale the building, reaching the roof and opening the door so his family can escape.
On the roof, they raise crude SOS signals to attract a passing rescue helicopter. But Yong-nam and Eui-joo are kept off the gondola by the weight limit, forcing them to improvise. They wrap themselves in improvised safety gear and move to higher ground, seeking another route to safety. They locate a suitable building and signal to a second helicopter, but their attention is drawn to a group of students trapped in a nearby building. They redirect the helicopter toward the students, sacrificing their own chance at rescue for others.
A nearby gas station explodes, sending a wave of gas their way and pushing them to leave the building before help can reach them. Realizing that the only escape is to reach the tower crane—the highest point in the vicinity—they chase after a route across multiple buildings, guided by a drone that seems to be their only clue.
The drone eventually loses power, leaving them at a dead end. Yet the city’s drones converge on their location, and Yong-nam manages to suspend a rope across the gaps by using one of them. The two attempt to swing between buildings, but their combined weight proves too much for the drone, and they seem to fall to their deaths. Miraculously, they survive the fall and reach the tower crane, from which they are rescued by a helicopter.
Back on solid ground, Yong-nam tearfully reunites with his family. As he and Eui-joo face each other, they share a quiet, hopeful moment and agree to see one another again one day. Rain begins to descend, slowly washing away the last of the white gas that once cloaked the city.
