In 1996, treasure hunter Brock Lovett and his team aboard the research vessel Akademik Mstislav Keldysh search the wreck of RMS Titanic for a necklace with a rare diamond, the Heart of the Ocean. They recover a safe containing a drawing of a young woman wearing only the necklace dated April 14, 1912, the day the ship struck the iceberg. Rose Dawson Calvert, the woman in the drawing, is brought aboard Keldysh and tells Lovett of her experiences aboard Titanic.
On July 2, a 500 km wide alien mothership enters Earth's orbit and deploys several dozen saucer-shaped "destroyer" spacecraft, each 15 miles (24 km) wide. As they take position over some of Earth's major cities, David Levinson (Jeff Goldblum), an MIT graduate working for a cable company in New York City, discovers hidden transmissions in Earth's satellites which he realizes is a timer counting down to a coordinated attack by the aliens. With the support of his estranged wife Constance Spano (Margaret Colin), the White House Communications Director, he and his father Julius (Judd Hirsch) gain entrance into the Oval Office to notify President Thomas J. Whitmore (Bill Pullman) about the attack. Whitmore orders large-scale evacuations of the targeted cities, but the aliens attack with advanced directed-energy weapons before these can be carried out. Whitmore, portions of his staff, and the Levinsons narrowly escape aboard Air Force One as Washington, D.C. is destroyed.
In 2009, American geologist Adrian Helmsley visits astrophysicist Satnam Tsurutani in India and learns that neutrinos from a massive solar flare are causing the temperature of the Earth's core to rapidly increase. Arriving at a party in Washington, D.C, Helmsley presents his information to White House Chief of Staff Carl Anheuser, who takes him to meet the President.
In the 2030s, the crew of Ares III, a manned mission to Mars, has established a temporary artificial habitat, called the Hab, on Mars, where they are to stay for two months and depart back to Earth in their orbital probe. Eighteen days into the Mars mission, however, a massive Martian storm hits the base and the crew is forced to abandon the planet and launch themselves because of the danger that their escape vehicle will collapse and leave them stranded. While evacuating, astronaut Mark Watney is lost and presumed dead as his biomonitor is damaged. With the lives of her crew at stake, mission commander Melissa Lewis is forced to leave the planet, leaving Watney behind.
The narrator (Morgan Freeman) explains how humans were unaware that intelligent extraterrestrials were making plans to occupy Earth. Ray Ferrier (Tom Cruise) is a divorced crane operator longshoreman who works at a dock in Bayonne, NJ. Ray is estranged from his children. His former wife, Mary Ann (Miranda Otto), later drops off the children, 10-year-old daughter Rachel (Dakota Fanning) and teenage son Robbie (Justin Chatwin), at Ray's house in Bayonne on her way to visit her parents in Boston. Unexplained changes in the weather occur, emitting lightning that strikes multiple times in the middle of an intersection and disrupting all electrical technology in the area.
A massive meteor shower destroys the orbiting Space Shuttle Atlantis and bombards a swath of land from America's East Coast from South Carolina through Finland. NASA discovers that a rogue asteroid the size of Texas passed through the asteroid belt and pushed forward a large amount of space debris. The asteroid will collide with Earth in 18 days, causing an extinction event that will even wipe out bacteria. NASA scientists, led by Dan Truman, plan to trigger a nuclear detonation 800 feet (240 m) inside the asteroid to split it in two, driving the pieces apart so both will fly past the Earth. NASA contacts Harry Stamper, considered the best deep-sea oil driller in the world, for assistance. Harry travels to NASA with his daughter Grace, to keep her away from her new boyfriend and one of Harry's drillers, A. J. Frost. Harry explains he will need his team, including A. J., to carry out the mission. They agree to help, but only after their list of unusual rewards and demands are met.
In June 1969, 5-year-old Jo Thornton (Alexa Vega), her parents, and the family dog Toby seek shelter in a storm cellar as a powerful F5 tornado. This tornado is so strong that the storm cellar door is ripped off, and Jo's father (Richard Lineback) is pulled into the storm to his death. Jo, her mother (Rusty Schwimmer), and Toby survive.
Raymond "Ray" Gaines (Dwayne Johnson) is a Los Angeles Fire Department Air Rescue pilot in the midst of a divorce from Emma (Carla Gugino) and planning a trip to San Francisco with his daughter Blake (Alexandra Daddario).
On July 20, 1969, astronaut Jim Lovell hosts a party, where guests watch on television as Neil Armstrong takes his first steps on the Moon during Apollo 11. After the party, Lovell, who had orbited the Moon on Apollo 8, tells his wife Marilyn that he intends to walk on the Moon's surface.
Los Angeles SWAT officers Jack Traven and Harry Temple thwart an attempt by a bomber to hold an elevator full of people for a $3 million ransom. They corner the bomber, but he grabs Harry. Jack shoots Harry in the leg, forcing the bomber to release him. He turns and runs around a corner, apparently dying in an explosion. Jack and Harry are praised by their superior, Lieutenant "Mac" McMahon.
On May 10, 1998, teenage amateur astronomer Leo Biederman discovers an unusual object near the stars Mizar and Alcor at a star party in Richmond, Virginia with his school's astronomy club. His teacher alerts astronomer Dr. Marcus Wolf, who realizes that the object is a comet on a collision course with Earth. Wolf dies in a car accident before he can alert the world.
In October 1991, the commercial fishing boat Andrea Gail returns to port in Gloucester, Massachusetts, with a poor catch. Desperate for money, Captain Billy Tyne (Clooney), convinces the Andrea Gail crew to join him for one more late season fishing expedition. The crew heads out past their usual fishing grounds in the Grand Banks, leaving a developing thunderstorm behind them. Initially unsuccessful, they head to the Flemish Cap, where their luck improves. At the height of their fishing the ice machine breaks; the only way to sell their catch before it spoils is to hurry back to shore. After debating whether to sail through the building storm or to wait it out, the crew decides to risk the storm. However, between the Andrea Gail and Gloucester is a confluence of two powerful weather fronts and a hurricane, which the Andrea Gail crew underestimates.
In the small town of Oakey Oaks, Chicken Little rings the school bell and warns everyone to run for their lives, allowing the whole town into a frenzied panic with much damage ensuing. Eventually, the Head of the Fire Department calms down enough to ask him what's going on and Little explains that a piece of the sky shaped like a stop sign had fallen on his head when he was sitting under the big oak tree in the town square; however, he is unable to find it. His father, Buck Cluck, assumes that this "piece of sky" was just an acorn that had fallen off the tree and had hit him on the head, making Little the laughing stock of the town.