The movie begins in a post-tsunami Hong Kong, now a submerged city teeming with dangerous creatures. The protagonist, Asha, has been living alone in an empty building for five years after the disaster. She struggles with isolation and scarcity of resources, managing her survival with meticulous care, including collecting rainwater and hiding from pirates.

One day, Asha spots a floating box and retrieves it from the water, finding only a bottle of whiskey inside, which she uses to clean a wound. Later, she encounters a group of robbers searching the building, which causes her to hide and reflect on happier memories with her family. Despite her efforts to stay safe, she is haunted by recurring nightmares about her family's drowning.