One morning after a violent thunderstorm, David Drayton (Thomas Jane), a graphic artist, and his wife Stephanie (Kelly Collins Lintz) check the damage. David decides to go to the local grocery store to buy supplies, bringing his eight-year-old son, Billy (Nathan Gamble), and his neighbor, Brent Norton (Andre Braugher) along. On the way, they see a convoy of military trucks. When they arrive at the store, they find it crowded with people who are also recovering from the storm. Suddenly, a tornado siren is heard and the power grid goes down, leaving the store with only a generator for the refrigeration units. As the town's tornado sirens go off, a panicked man with a bloody nose, Dan Miller (Jeffrey DeMunn), runs into the store warning of something dangerous in the oncoming mist, which quickly envelops the store, making it impossible to see outside.