Mathieu and Anna have a teenage daughter named Sarah who suffers from a serious illness. The disease she suffers, known as orphan’s disease, compels her to live isolated, in a kind of balloon that protects her from the dangers of the outside. One day, a deadly fog invades the city of Paris. Sarah’s parents take refuge on the top floor of the building where they live, hoping the mist will soon dissipate. But, unfortunately, things only get worse. Mathieu and Anna have to plan a survival plan to allow their daughter to leave their temporary ball. Encouraged by the immense love that their daughter bears, the two will venture into an empty and contaminated city full of dangers in an attempt to save all three.