It's early on the morning of September 11, 2001 and Control Sergeant John McLoughlin (NICOLAS CAGE) of the New York Port Authority has left his wife, Donna (MARIA BELLO), and kids in bed as he heads off for work. A 21-year-veteran, he has a number of rookies under his watch, including Will Jimeno (MICHAEL PENA), but the day looks routine, with the only advisory being for them to be on the lookout for a runaway headed for the city.
Then the unthinkable happens, as a plane crashes into one of the World Trade Center towers. John, Will and a number of other men, including Dominick Pezzulo (JAY HERNANDEZ) head down there, unsure of what's happening, and don't believe rumors that another plane has struck the other tower while they're en route.
But when they get there, chaos reigns and they immediately go into action, grabbing firefighting gear in the concourse level between the towers. Before they can make their ascent, however, the first tower collapses, burying John and his men under an enormous amount of debris.
With only three of them surviving but unable to get out, John and the other survivors must contend with their injuries, additional collapses, as well as the passage of time. Their respective families -- including Will's pregnant wife Alison (MAGGIE GYLLENHAAL) -- anxiously await any sort of word about their whereabouts and condition.
Meanwhile, ex-Marine Dave Karnes (MICHAEL SHANNON) travels to the city and proceeds on a one-man rescue mission, driven by his calling from God and his belief that this is the only thing to do. As other rescuers arrive on the scene, including former paramedic Scott Strauss (STEPHEN DORFF), the trapped men try to hold on as long as possible, while their loved ones hope and pray for the best.