[4] The film stars Romain Duris and Olga Kurylenko as Mathieu and Anna, parents trying to protect their daughter (Fantine Harduin) from a toxic gas cloud that is enveloping Paris.

[6] Parisian girl Sarah is one of a group of children who has a mysterious 'total allergy' auto-immune syndrome that forces them to live in hermetically-sealed capsules, while her parents, scientists Mathieu and Anna, desperately search for a cure.

A while later, there is an apparent earthquake, followed by the release of a deadly gas which emerges from the ground and begins to spread through the streets, killing everyone who inhales the toxic fumes.

Fortunately, the gas proves to be heavier than air, and Mathieu and Anna find refuge above the mist in the top-floor apartment of their elderly neighbors, Lucien and Colette.

Sarah is forced to remain in her bubble in the family's now fog-filled flat, which has now been only limited to battery power mode to keep her air filtration system operating.

The next day, Mathieu and Anna witness riots breaking out among the survivors at Montmartre and decide to escape to the house of Sarah's friend Charlotte, who lives in the hills.

As the fog rises further and seeps into the apartment, Mathieu decides to get a hazmat suit he saw earlier in a crashed car and adapt it for Sarah, while Lucien and Colette lie on their bed and await death.