In May 1997, at a star party, teenage amateur astronomer Leo Beiderman discovers an unusual object. He sends a picture to astronomer Dr. Marcus Wolf, who realizes it is a comet on collision course with Earth. Wolf dies in a car crash while racing to raise the alarm.
A year later, journalist Jenny Lerner investigates Secretary of the Treasury Alan Rittenhouse over his connection with "Ellie", whom she supposes to be a mistress. She is abducted by the FBI to meet President Tom Beck, who persuades her to sit on the story for 48 hours in return for a prominent role in the press conference he will arrange. She subsequently discovers that "Ellie" is actually the acronym "E.L.E.", for "extinction-level event". Two days later, Beck announces that the comet Wolf-Beiderman is headed for Earth and could cause humanity's extinction. He reveals that the United States and Russia have been constructing the Messiah in orbit, a spacecraft to transport a team to alter the comet's path with nuclear bombs.
While rigging nuclear bombs on the comet one astronaut is blinded and another flung into space. The ship is damaged by the blast and loses contact with Earth. Beck announces the mission's failure, the bombs split the comet into a larger (Wolf) and a smaller (Beiderman) piece, both still headed for Earth. Martial law is imposed and a lottery selects 800,000 Americans to join 200,000 pre-selected individuals in underground shelters. Lerner and the Beiderman family are pre-selected, but Leo's girlfriend Sarah and her family are not. Leo marries Sarah in a vain attempt to save her family, and Sarah refuses to go to the shelter without them.
A last-ditch effort to deflect the comets with ICBMs fails. Leo eschews his safety to find Sarah and takes her and her baby brother to high ground. Lerner gives up her evacuation helicopter seat to her friend Beth and her young daughter, reconciling with her estranged father on the beach. Comet Beiderman creates a megatsunami that destroys much of the East Coast of the United States and later Europe and Africa as well, resulting in millions of fatalities. Leo, Sarah, and her baby brother survive on the Appalachian Mountains.
The crew of Messiah decide to obliterate Wolf by undertaking a suicide mission. After saying goodbye to their loved ones, they fly the ship into a deep crevasse and use their remaining nuclear warheads to blow Wolf into smaller pieces that burn up in Earth's atmosphere.
After the waters recede, President Beck speaks to a large crowd at the damaged United States Capitol, encouraging them to remember those lost as they begin to rebuild.
Robert Duvall as Captain Spurgeon "Fish" Tanner,
Téa Leoni as Jenny Lerner,
Elijah Wood as Leo Beiderman,
Vanessa Redgrave as Robin Lerner
Maximilian Schell as Jason Lerner,
Morgan Freeman as Tom Beck,
James Cromwell as Alan Rittenhouse,
Ron Eldard as Commander Oren Monash,
Jon Favreau as Dr. Gus Partenza,
Laura Innes as Beth Stanley,
Bruce Weitz as Stuart Caley,
Mary McCormack as Andrea "Andy" Baker,
Richard Schiff as Don Beiderman,
Betsy Brantley as Ellen Beiderman,
Leelee Sobieski as Sarah Hotchner,
Blair Underwood as Mark Simon,
Dougray Scott as Eric Vennekor,
Mark Moses as Tim Urbanski,
Aleksandr Baluev as Colonel Michail Tulchinsky,
Mike O'Malley as Mike Perry,
Francis X. McCarthy as General Scott
Kurtwood Smith as Otis "Mitch" Hefter,
O'Neal Compton as Morten Entriken,
Kimberly Huie as Wendy Mogel,
Denise Crosby as Vicky Hotchner,
Christopher Darga as Section Leader