In a quiet and dark winter, just before Christmas, fog suddenly fell in the city of Ryan where Susan was located. Schools were suspended, factories were suspended, and the stock market was suspended... The poisonous gas caused by an explosion leaked into the haze, plunging the entire city into panic. Susan's family of six was forced to hide at home to avoid the attack of poisonous fog. Faced with these changes, love, family, and friendship have become so fragile. For a moment, the end seemed to be coming... But this was just the beginning. The crisis caused social unrest, and everyone was doing whatever it took to survive. Susan received mysterious warnings, terrifying nightmares and visits from strangers, all of which made her nervous. When the poisonous gas attacks and her home can no longer be used as a shelter, she has no choice but to leave here and seek a slim survival, but there is a dark conspiracy behind it... Betrayal, intimidation and despair, when everyone around her falls one by one, where should Susan go...