In 2009, Dr. Adrian Helmsley, an American geologist visits astrophysicist Dr. Satnam Tsurutani in India and learns that neutrinos from a massive solar flare are causing the temperature of the Earth's core to increase rapidly. Adrian informs White House Chief of Staff Carl Anheuser and United States President Thomas Wilson that this will trigger a catastrophic chain of natural disasters. In 2010, Wilson, along with other international leaders, begins a massive, secret project intended to ensure that humanity survives. Approximately 400,000 people are chosen to board ships called "arks" that are constructed at Cho Ming, Tibet in the Himalayas. Additional funding for the project is raised by selling tickets to the private sector for €1 billion per person. By 2011, they start to secretly move humanity's valuable treasures to the Himalayas under the guise of protecting them from terrorist attacks, so that their history can survive when the end comes.
In 2012, Jackson Curtis is a science fiction writer in Los Angeles who works part-time as a limousine driver for Russian billionaire Yuri Karpov. Jackson's ex-wife Kate and their children Noah and Lily live with Kate's boyfriend, plastic surgeon, and amateur pilot, Gordon Silberman. Jackson takes Noah and Lily camping in Yellowstone National Park, where they meet Charlie Frost, who hosts a radio show from the park. Charlie references a theory that suggests the Mesoamerican Long Count Calendar predicts that the 2012 phenomenon is going to occur. He has a map of the ark project in addition to information of government officials and scientists from around the world who were murdered after discovering information about the ark project and trying to alert the general public. The family returns home as seismic activity vastly increases along the US West Coast. Jackson grows suspicious and rents a plane to rescue his family. He collects his family and Gordon as the Earth crust displacement begins, and they narrowly escape Los Angeles using a Cessna 340 as the city sinks into the Pacific Ocean.
As millions die in cataclysmic earthquakes worldwide, the group flies to Yellowstone to retrieve Charlie's map, escaping as the Yellowstone Caldera erupts. Remaining at his site to broadcast the eruption, Charlie is killed in the blast. Learning the arks are in China, the group lands in Las Vegas, which has a huge fissure running through the airport's runway. Here, they meet Yuri, his twin sons, girlfriend Tamara and pilot Sasha. The group secures the Antonov 225 aircraft and departs for China. Also heading for the arks aboard Air Force One are Anheuser, Helmsley, and First Daughter Laura Wilson. President Wilson chooses to remain in Washington D.C. and is soon killed by a megatsunami which sends the USS John F. Kennedy crashing into the White House. With the Vice President also dead and the Speaker of the House missing, Anheuser assumes de facto leadership of the United States of America, although he is not in the line of succession.
Arriving in China in a crash landing that kills Sasha, the group is then spotted by the Chinese People's Liberation Army. Yuri and his sons, possessing tickets, are taken to the arks. The Curtis family, Gordon, and Tamara, none of whom possess tickets, are picked up by Nima, a Buddhist monk on his way to the arks. They stowaway with the help of Nima's brother, Tenzin, who has been working on the ark project. A megatsunami approaches the site as tens of thousands of people are attempting to board the final ark when a large impact driver becomes lodged between the gears of the ark's hydraulics chamber, preventing a boarding gate from closing and rendering the ship unable to start its engines. In the ensuing chaos, Yuri, Gordon, and Tamara are killed, Tenzin is wounded and the flooding ark is set adrift. Jackson and Noah free the impact driver from the closing mechanism, and the crew regains control of the ark, preventing a fatal collision with Mount Everest.
One month later, after floodwaters from the tsunamis have receded, three arks set sail for the Cape of Good Hope in South Africa, where the Drakensberg Mountains have risen in relation to sea level. Jackson reconciles with his family and Adrian starts a relationship with Laura. The Earth is shown from space, revealing a radically changed continental landscape.