The film is inspired by the idea of a global doomsday event coinciding with the end of the Mayan Long Count Calendar's current cycle on or around December 21, 2012(the northern hemisphere's winter solstice).
Jackson Curtis (John Cusack) is a divorced father who occasionally works as a limousine driver and a writer, while his ex-wife Kate Curtis (Amanda Peet) and children live together with her new boyfriend, Gordon (Thomas McCarthy). At the Mayan city of Tikal in Guatemala, the victims of a mass suicide seem to adhere to the Mayan calendar, which predicts the end of the world will coincide with a Galactic Alignment, which is to occur on December 21 of 2012, the date of the northern hemisphere'swinter solstice.
The IHC (Institute for Human Continuity), a secret organization, has realized the situation and begins constructing vast arks beneath the Himalayan mountains designed to withstand most natural disasters in order to save both humanity, significant species, and the greatest treasures of mankind for when doomsday finally happens.
There are arguments however about how and when the governments of the world will alert their fellow citizens and even discussions about how the remaining survivors will be selected to survive Armageddon, but when they discover that the global cataclysm is happening much faster than expected, they must race to these ships to save the human race before all is lost.
Meanwhile on a day trip to Yellowstone with his two young children, Jackson meets Charlie Frost, who hosts his own radio segment and blog about his belief that the Mayans are correct about 2012.
Over the coming days vast cracks are found within the fault lines in California, and despite the pleas from the government that everything is fine, Jackson isn't convinced. Hiring a private plane and gathering crucial supplies he races to Kate's home to save himself and his family from the Earth's crust displacement occurring all around them.
After a long and dangerous race through the collapsing streets to get to the Santa Monica Airport, Gordon uses his pilot skills to get the family to safety as the runway crumbles beneath them. After they take off, they fly dangerously through Los Angeles as the entire city collapses into oblivion. As they look back, the entire cityscape has already started to crumble into the Pacific.
As the plane eventually runs low on fuel, the group finds itself obligated to land in Wyoming in order to refill. Jackson uses this opportunity to get in contact with Charlie. Against Kate's will, Jackson runs off with his daughter towards Charlie's van. Upon his arrival, he finds an empty van and soon realizes that Charlie has left. However, Charlie informs his followers on the radio that he has decided to go into the mountains to watch the imminent catastrophe. Jackson takes the van and speeds off to find him. His pursuit proves to be successful, but the ground below them begins to rumble. Jackson warns Charlie that they must leave, yet he refuses as he finds the scenery "too beautiful." Before Jackson escapes, Charlie informs him that he has a map in his van that will provide information concerning a possible escape route. As the volcanoes erupt behind him, he heads back towards the plane.
Again, Jackson finds himself on a dangerous race as the large chunks of boulders strike the ground nearby. Upon his arrival, his daughter charges out of the van into the plane, but Jackson remains in the van to search for the map. He recalls that Charlie told him it was "between Roswell and Marilyn Monroe," yet he takes too long to find it. As the earth opens up, the van falls into a gap. The family is horrified, but must leave before it is too late. The ground is continually crumbling closer and closer. Meanwhile, Jackson finds himself falling but manages to grab onto the ledge. He pulls himself back up and runs back to the plane as the ground behind him continues to collapse. He is just fast enough to enter the plane.
It soon becomes clear that it's not just California that's experiencing disaster. The Yellowstone Supervolcano erupts, Las Vegas is destroyed by a massive chasm that cuts through the Strip and by an ash cloud from the Yellowstone eruption, massive earthquakes begin to occur in South America (destroying the cities of Rio de Janeiro and Buenos Aires), Washington D.C. is flooded by an enormous tsunami in which the USS.John F. Kennedy crushes the White House and St. Peter's Basilica in Rome crumbles into oblivion, killing thousands in prayer. As massive tidal waves began to sweep across the Earth, the U.S. government declares the end of the world.
A group of survivors, including Jackson and his family, must fight their way to China before they can board the great arks and save themselves from the gigantic tidal waves flooding the Earth. They still find themselves in the plane, but realize it will not suffice to fly all the way to China. They agree that they must land at an airport to find a better plane. While they manage to find one, it is filled with many other individuals. However, Gordon stumbles upon one of his old clients, Tamara (Beatrice Rosen). She is with Russian millionaire, Yuri Karpov (Zlatko Buric). Upon closer investigation, the family realizes that Yuri has bought a plane to escape to China. Jackson pleads him to allow his family, but Yuri initially refuses. However, his pilot Sasha (Johann Urb) alerts him that he requires a co-pilot. Jackson seizes this opportunity and tells Yuri that Gordon is an experienced pilot. Subsequently, the group board a large Russian plane towards China. Below them, the airport is destroyed by earthquakes.
As they are in the air, Sasha realizes that the plane does not contain sufficient fuel to fly all the way to China. He warns Gordon of this and both agree that they will have to land in the water. Gordon proceeds to tell this to Jackson. They wake up up the others on board to warn them. Kate and Jackson hand life jackets to their children as they prepare themselves for the landing. Meanwhile, Sasha discovers that they are no longer above the ocean. The Earth has shifted by thousands of miles and they are heading right towards the Himalayas. The group must rapidly concoct a new plan. They all decide to go into cars while Sasha opens the door from the cockpit. Yet time is running out and the plane is charging right into the mountains. Gordon must abandon Sasha and runs back to the car just in time to escape with the others. However, Sasha lands the plane on a cliff. The weight of the plane causes the ground below it to break leading Sasha to his death.
The remaining group members has managed to land safely with the cars. Tamara weeps and demands they go back to find Sasha. Yet before they have time to act, Chinese helicopters holding large animals, such as elephants and giraffes, fly above them. One helicopter notices the group and goes down to greet them. However, the group immediately understand that they will need to pay to get into the helicopter. Yuri pays for him and his sons, but refuses to pay for the others. Before boarding the helicopter, he tells Tamara he knew of her relationship with Sasha.
The group now has no choice but to walk through the mountains in an attempt to find others. Soon enough, a car drives by them. In it are Buddhist monk, Nima (Osric Chau), and his grandmother (Lisa Lu). While they first drive past the group, Jackson throws a rock at it. The car backs up and they allow them to join along for the ride to meet Nima's brother Tenzin (Chin Han). He has a plan to escape.
Upon their arrival, Tenzin is infuriated. He tells Nima his plan could not account for so many people and that, therefore, the others would not be able to join along. Nonetheless, Jackson and Kate insist that they take the children. After further discussion, Tenzin allows the entire group to join. His plan was to sneak into an ark that had been created by governments around the world as a way to save a select few.
Curtis and family managed to sneak into the ark with the help of Tenzin. Anheuser, who is now in charge of the ark, ordered that the gates of the arks to be closed as the supplies are not enough to cater for all the survivors. Helmsley, dismayed at Anheuser's action, convinced the leaders of various nations to open the gates to save the survivors.
When closing the gates, part of Tenzin's leg is crushed. Gordon also died though Curtis made an attempt to save him. The cable brought by Tenzin was stuck in between the gears and the gears are grind to a complete halt. The gate is now partially closed. As the tsunami hits the ark, one of the main supporting structure of the ark breaks down the the ark is now floating in the direction of Mount Everest. The engines of the ark cannot be started even with manual settings unless the gate closes.
Curtis decided to give it a try and remove the cable from the gear with the aid of his son. He succeeded. The gears are operating again and the gate closes. The engines are activated and are engaged in a reverse position to avoid an impact. However, it still hit Mount Everest but with minimal damage. The survivors inside the ark cheers for Curtis who managed to save them.
When the catastrophy ended, the water levels subsided. The captain of the ark along with the sister arks decided that Cape of Good Hope in South Africa is a good place to lead a normal life. The survivors inside walks out from the ark and are able to see the sunlight and the vast sea again. The film ends with the sight of the African continent, capable of supporting human life in the future.
