On September 11, 2001, Port Authority Police Officers John McLoughlin and Will Jimeno are patrolling the Port Authority Bus Terminal in Midtown Manhattan when they see a plane fly dangerously low overhead. As all of the police return to the station, they see on TV that the North Tower of the World Trade Center has been hit by the plane. Sergeant McLoughlin assigns many of the officers to assist in a precautionary evacuation attempt of the North Tower and they board a Metropolitan Transit Authority bus. On the bus, they hear reports that the South Tower is also hit by another plane. When they arrive at the World Trade Center, they realize the extent of the disaster, and see one of the victims jump out of the towers to certain death. The men proceed to get safety equipment from Building 5 and enter the concourse between the towers.
The group consists of McLoughlin, Jimeno, Dominick Pezzulo, and Antonio Rodrigues. Officer Christopher Amoroso appears to inform them of other events, such as the American Airlines Flight 77 attack, the second plane's hit on the South Tower, and an attack on Israel, though the group does not accept any of these as true. As the men prepare to enter the North Tower, the buildings begin to rumble. McLoughlin realizes that the South Tower is collapsing onto them and that their only chance of survival is to run into the service elevator shaft. Amoroso trips and does not have time to get up, and Rodrigues is unable to get to the shaft in time, resulting in both deaths. McLoughlin, Jimeno, and Pezzulo manage to escape the huge amounts of dust and rubble flying down from the South Tower. However, as the rubble continues to crush the elevator shaft, the three are trapped. As the cascade of debris subsides, Pezzulo realizes he can free himself and manages to move closer to Jimeno in order to shift the debris covering his legs, but cannot make it to McLoughlin as he is trapped deeper in the rubble. Pezzulo tries but fails to shift the debris due to its weight, and is instructed by McLoughlin not to leave.
Shortly after word is known that a fourth plane, United Airlines Flight 93, has been hijacked and has crashed in Shanksville, Pennsylvania.
As Pezzulo becomes optimistic that they will live, the rumbling begins again as the North Tower starts to collapse. Although Jimeno and McLoughlin are not further harmed, Pezzulo is fatally injured when a concrete slab falls into the hole, crushing his torso. After he fires a gun through a gap in the rubble to try to alert rescuers to their position, he dies. Jimeno and McLoughlin spend hours under the rubble, in pain but exchanging stories about their lives and families. McLoughlin is particularly anxious to keep Jimeno from falling asleep and Jimeno also realizes that by straining to grab a metal bar above his body, he can make a noise that rescuers might hear. Two United States Marines, Dave Karnes and Jason Thomas, who are searching for survivors, do hear it and find the men, calling for help to dig them out. Jimeno is rescued first, and then hours later McLoughlin is lifted out of the debris, barely alive and in critical condition. They are then both reunited with their distraught families at the hospital. Two years after the attacks, McLoughlin and Jimeno attend a barbecue with their families: McLoughlin's wife Donna, Jimeno's wife Allison, daughter Bianca, and their newest addition Olivia.
The epilogue states that John and Will were 2 of the 20 people pulled out alive and are now retired from active duty. Dave Karnes re-enlisted in the Marines.
Nicolas Cage as John McLoughlin
Michael Peña as Will Jimeno
Maria Bello as Donna McLoughlin
Maggie Gyllenhaal as Allison Jimeno
Stephen Dorff as NYPD ESU Rescueman Scott Strauss
Jon Bernthal as Officer Christopher Amoroso
Jay Hernandez as Officer Dominick Pezzulo
Michael Shannon as Marine Sgt. Dave Karnes
Donna Murphy as Judy Jonas
Brad William Henke as Allison's brother
Peter McRobbie as Allison's father
Frank Whaley as Paramedic Chuck Sereika
Jude Ciccolella as Inspector Lawrence Fields
Nicholas Turturro as Officer Colovito
Danny Nucci as Officer Giraldi
William Mapother as Marine Sgt. Jason Thomas
Wass Stevens as Officer Pat McLoughlin
Armando Riesco as Officer Antonio Rodrigues
Ned Eisenberg as Officer Polnicki
Dara Coleman as Officer Boel
Nick Damici as Lt. John Kassimatis
Arthur Nascarella as FDNY Chief at Ground Zero
Patti D'Arbanville as Donna's Friend
Viola Davis as Mother in Hospital
Will Jimeno as Port Authority Police Officer
Aimee Mullins as Reporter
Charles A. Gargano as Port Authority Police Commander