On the morning of September 11, 2001, four al-Qaeda terrorists Ziad Jarrah, Saeed al-Ghamdi, Ahmed al-Nami, and Ahmed al-Haznawi pray in their respective hotel rooms before arriving at Newark Liberty International Airport. The quartet wait at a gate after getting through security to board San Francisco-bound United 93. After 40 minutes of delay due to traffic the plane takes off, flown by captain Jason Dahl and first officer Leroy Homer, with all four terrorists on board.
An earthquake strikes the city of Los Angeles. Mike Roark (Tommy Lee Jones), the head of the city's Office of Emergency Management, insists on coming to work to help out with the crisis, even though he has taken a vacation with his daughter Kelly (Gaby Hoffmann). His associate Emmit Reese (Don Cheadle) notes the quake caused no major damage, but seven utility workers are later burned to death in a storm drain, one escaped and survived but was severely burned on one side of his face at MacArthur Park. As a precaution, Roark tries to halt the subway lines which run parallel to where the deaths took place, but Los Angeles MTA Chairman Stan Olber (John Carroll Lynch) declines, feeling there is no threat to the trains. Against regulations, Roark and a colleague Gator Harris (Michael Rispoli) venture down the storm sewer in the park to investigate. They are nearly burned to death when hot gases suddenly spew out of a crack in the concrete lining and flood the tunnel. Geologist Dr. Amy Barnes (Anne Heche) believes a volcano may be rapidly forming beneath the city with magma flowing underground (similar to the formation of the Mexican volcano Parícutin which emerged and grew tremendously in just one week) but cannot come up with enough evidence for Roark to take action.
Vulcanologist Peter Slater Dan Cortese, studying elevated steam-activity on Angel Mountain, finds evidence of an impending eruption such as many animals lying dead for no apparent reason. His ex-girlfriend Kelly Adams, a resident of the eponymous town, is told about his findings—which unfortunately, happen to be during the peak tourist season. Kelly tries in vain to warn the townsfolk and council of the danger, but they fail to heed and evacuate—until the mountain shows ashfall. After several attempts to find a solution, Peter declares that the lava is soon going to come down like "a liquid avalanche" which gives Kelly the idea (due to Angel Mountain and some others nearby being snowcapped) of using a snow avalanche to stop the lava flow—at which Peter balks, until Kelly tells him of the lack of other albernatives. The two then set out skiing to planting explosives to set off a controlled avalanche to stop the flow—but in attempting to escape the resulting avalanche Peter gets buried and Kelly makes it into town to witness the result, the snow avalanche blocking off (and solidifying) the lava-flow. At that point, Peter manages to dig himself out and return to town, after which he and Kelly rekindle their romance.
Broadcast eleven years after a similar program, Special Bulletin, Without Warning starts in an identical fashion, with the beginning of "regular programming", in this case the opening of a murder mystery film with the title Without Warning, starring Loni Anderson (appearing in a cameo). Within moments, however, the program is interrupted with a news bulletin of an earthquake in Wyoming. The "movie" resumes but a few moments later is interrupted for good as coverage begins of a Halloween night meteor impact on the United States.
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 officers 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.
Ex-fighter pilot and taxi driver Ted Striker (Robert Hays) became traumatized during an unnamed war, leading to a pathological fear of flying. As a result, he is unable to hold a responsible job. His wartime girlfriend, Elaine Dickinson (Julie Hagerty), now a flight attendant, leaves him. Striker nervously boards a Boeing 707 (Trans American Flight 209) from Los Angeles to Chicago on which she is serving, hoping to win her back, but she rebuffs him.