Paris in shadowed quiet gives way to a chilling premise: a group of children, led by Sarah, live with a mysterious “total allergy” autoimmune syndrome that confines them to hermetically sealed capsules, while their parents, scientists Mathieu, Romain Duris and Anna, Olga Kurylenko, race against time to find a cure. A glimmer of hope arrives when Mathieu returns from Canada with news of a possible breakthrough, and the family clings to that possibility as the city around them seems to unravel.
Then disaster strikes in the form of an earthquake followed by the release of a deadly gas that pours from the earth and begins to choke the streets. The fumes are heavier than air, and those who inhale them perish, while those who can reach higher ground survive a bit longer. The family seeks safety above the fog in the top-floor apartment of their elderly neighbors, Lucien, Michel Robin, and Colette, Anna Gaylor. Sarah remains sealed inside her bubble, her life sustained by a battery-powered air filtration system as the building fills with the ominous mist.
Mathieu, driven by a fierce sense of responsibility, braves a perilous trek into the lower apartment. By holding his breath, he slips through a barrier of danger, steals an oxygen tank and mask, and returns with supplies that keep Sarah alive and her battery charged. On the way, he wanders the corpse-strewn streets and encounters a convoy of survivors led by soldiers in gas masks, bound for Montmartre, a higher refuge. The soldiers offer him two new masks and tanks before he is forced to continue his perilous journey alone. He discovers the fog creeping upward, threatening to close the distance between him and his family.
With riots erupting in Montmartre, Mathieu and Anna decide to escape to the hills where Charlotte, Erja Malatier, a friend of Sarah who shares the same syndrome, resides. The plan hinges on getting Sarah into a sealed suit, a dangerous and delicate operation that requires a trip to a laboratory. The escape is messy: Mathieu is separated when a dog pursues him and he falls into the Seine, but he reunites with his wife at the laboratory. After obtaining a suit, an explosion damages Mathieu’s oxygen tube, forcing them to share Anna’s oxygen as they maneuver toward an office above the fog. Yet Anna’s supply is not enough for both to descend to safety, so she returns alone with the suit while Mathieu attempts to reach safety by leaping rooftops.
In a deserted parking garage high above the haze, Mathieu discovers a cache of supplies but finds the space policed by a suspicious officer—the Drunk Policeman, Alexis Manenti. Realizing the cop won’t let him claim the goods, Mathieu is driven to a desperate act. He ends the threat, but not without consequences. Back at the apartment, Anna’s oxygen runs dangerously low, and with Sarah’s battery depleted, she makes a final, heartbreakingly brave attempt to reach her sister’s stairwell before blacking out; Sarah collapses before Mathieu can reach her.
As the fog thickens and infiltrates the apartment, Mathieu resolves to rescue Sarah by donning a hazmat suit he spots in a crashed car and adapting it for her. Lucien and Colette lie on their bed, waiting for the end. Mathieu’s journey back is nearly thwarted when he nearly collides with a masked boy, but the collision becomes a turning point: the boy is Noé, Sarah’s boyfriend, who shares the same condition as Charlotte, and he is guiding Sarah through the fog. Noé, Réphaël Ghrenassia, appears with a sense of calm and resilience that ultimately reveals a startling truth—the fog does not affect those who carry the same disease. Reunited, Sarah and Noé stride through the mist, while Mathieu, exhausted and overwhelmed, slips into unconsciousness.
When he awakens, the world has shifted: he finds himself back in Sarah’s medical capsule, with Sarah now taking care of him. The balance of care and danger has flipped, turning a desperate search into a reversal of roles as the family clings to one another in a world where the fog’s grip remains uncertain, and hope takes on a new, intimate form.
