In 2009, American geologist Adrian Helmsley visits astrophysicist Satnam Surtani in India and learns that neutrinos from a huge solar flare are heating Earth's core. Helmsley presents his information to White House Chief of Staff Carl Anheuser, who brings him to meet the president.
The following year, U.S. President Thomas Wilson and other world leaders begin a secret project to ensure the survival of humanity. China and the G8 nations begin building nine arks, each capable of carrying 100,000 people, in the Himalayas near Cho Ming, Tibet. Nima, a Buddhist monk, is evacuated and his brother Tenzin joins the ark project. Funding is raised by selling tickets at âŹ1Â billion per person. By 2011, articles of value are moved to the arks with the help of art expert and First Daughter Laura Wilson.
In 2012, struggling Los Angeles science-fiction writer Jackson Curtis is a chauffeur for Russian billionaire Yuri Karpov. Jackson's former wife (Kate) and their children (Noah and Lilly) live with Kate's boyfriend, plastic surgeon and pilot Gordon Silberman. Jackson takes Noah and Lilly camping in Yellowstone National Park. When they find an area fenced off by the Army, Jackson and his children climb over the fence. They are caught and brought to the geologist Adrian, who has read Jackson's books. After they are released they meet Charlie Frost, who hosts a radio show from the park. After Jackson and his children leave, Adrian learns that the Earth is about to undergo drastic changes in a few hours, realizing that it came much earlier than expected, as only three of the arks have been completed by this time.
That night, after the military evacuates Yellowstone, Jackson watches Charlie's video of Charles Hapgood's theory that polar shifts and the Mesoamerican Long Count calendar predict a 2012 phenomenon. According to Charlie, anyone who attempted to inform the public was killed. Soon after Jackson and his children return home, he delivers Yuri's sons Alec and Oleg to the airport. After the two express that "they will live and he will die", Jackson rents a plane and rushes back to his family, realizing that Charlie was right. He rescues his family as the Earth-crust displacement begins and escapes from Los Angeles by air as California sinks into the Pacific Ocean.
They fly to Yellowstone to retrieve a map from Charlie with the arks' location. As they leave, the Yellowstone Caldera erupts; Charlie is killed when he stays behind to cover the eruption. The group lands in Las Vegas to find a larger plane and meet Yuri, his twin sons Alec and Oleg, his girlfriend Tamara and their pilot Sasha, who are unable to leave because the airport is refusing to let them. Sasha an Antonov An-500 and pilots it with Gordon, and leave just as the Yellowstone ash cloud engulfs Las Vegas.
Adrian, Carl and Laura fly to the arks on Air Force One. Knowing that his daughter will survive, President Wilson remains in the capital to address the nation for the last time while millions of people die in earthquakes and megatsunamis worldwide. With the presidential line of succession gone, Carl assumes the position of acting commander-in-chief. In the Pacific Ocean, a megatsunami strikes the Genesis and kills everyone on board, including Adrian's father.
Yuri informs Jackson that he plans to stop at Hawaii to refuel, but this is cut short when they find that Hawaii has been engulfed in lava, forcing them to continue without the needed fuel, and arrive at China. Sasha continues flying the plane as the others escape on a Bentley Continental Flying Spur stored in the cargo hold. Sasha is killed when the plane crashes, and the others are spotted by Chinese Air Force helicopters. Yuri and his sons (who have tickets) are brought to the arks and the Curtis family, Tamara and Gordon are left behind. The remaining group is picked up by Nima and brought to the arks with his grandparents. With Tenzin's help they stow away on Ark 4, where the U.S. contingent is located. As a megatsunami breaches the Himalayas and approaches the site, an impact driver lodges in the ark-door gears. This keeps a boarding gate open, which prevents the ship's engines from starting. In the ensuing chaos where many people attempt to board the ships, Yuri, Tamara and Gordon are killed. Tenzin is injured, the ark begins filling with water and is set adrift. Carl wants to leave immediately, but Adrian manages to persuade the heads of state to let the people outside in. Ark 4 attempts to leave, but the lodged impact driver prevents it from doing so. Jackson and Noah dislodge the tool, and the crew of Ark 4 barely manages to regain control of the Ark before it smashes into Mount Everest. Jackson is reunited with his family and reconciles with Kate.
Twenty-seven days later, the waters are receding. The arks approach the Cape of Good Hope, where the Drakensberg (now the tallest mountain range on Earth) is emerging. Adrian and Laura begin a relationship, and Jackson and Kate rekindle their romance.

Question: What prevents Ark 4's engines from starting?
Answer: An impact driver is lodged in the ark-door gears.
Alternative answer: A boarding gate being open.