Seven Tulane University undergraduates – Sara, Nick, Beth, Malik, Maya, Blake and Gordon – drive to Sara's family vacation home on a private lake. There, Sara encounters her old boyfriend, Dennis and his friend, Red. Nick, Blake, Malik and Maya go wake-boarding when a shark attacks Malik. He is able to swim back to shore – missing his right arm. When Nick swims into the lake to retrieve Malik's arm, he is pursued by the shark, barely making it to shore. Being a pre-med student, he stabilizes Malik, and they all take him to the hospital. Blood from Malik's wound attracts a shark that attacks the boat. Maya (Malik's girlfriend who he was about to propose to) is knocked into the water and eaten. The shark damages the steering column of the boat, causing it to crash in front of the boathouse. Sara, Nick and Malik make it to shore. Dennis and Red arrive and agree to take Beth and Gordon to the mainland. During the boat trip, Dennis reveals that he and Red put the sharks in the lake and feed college students to them. Dennis forces Gordon into the water by shooting him; he swims to a mangrove tree but is devoured by a bull shark. Red and Dennis force Beth to strip down and feed her to cookiecutter sharks. Malik arms himself with a harpoon and ventures into the water to kill the shark that killed Maya. His wound attracts a hammerhead shark; he kills it but is further wounded. Nick and Blake pull the shark to shore, and Nick finds a camera attached to its belly. Blake decides to take Malik to the mainland on a jet ski. When they are pursued by sharks, Malik sacrifices himself and later Blake is killed by a Great white shark. Sheriff Sabin visits Sara and Nick and gives them soup, which makes Nick and Sara's dog, who has also eaten the soup, pass out. Sara hears Dennis and Red speaking from Sabin's radio, describing Beth's death. Dennis and Red then appear and take Sara to their boat, while Sabin prepares to dunk Nick into the lake, where sand tiger sharks wait. Sabin reveals that he, Dennis and Red have been inspired by the Discovery Channel's Shark Week and Faces of Death videos to film people being eaten by sharks. Nick is able to break free and ignite a can of gas. Badly burned, Sabin plunges into the lake and is eaten by the tiger sharks. Dennis reveals he is bitter at Sara for running him over with a jet ski, damaging half his face a while back. Red throws her dog into the water and soon after Dennis calls up Carl, the guy from the bait shop, telling him to release another great white shark, Dennis and Red lower her into the lake in a shark cage. As Dennis is about to release her, Nick arrives and takes Red hostage. After a scuffle during which Dennis kills Red in order to catch Nick offguard, Nick shoves Dennis into the water and tries to set Sara free but Dennis appears and tries strangling him. Nick escapes as a great white shark devours Dennis. Nick kills the shark and frees Sara. Nick, Sara, and Sara's dog swim to the boat, having survived. A distance away, a great white shark breaches, signifying that sharks remain in the lake

In Philadelphia, Detective Bowden is still grieving for his wife and son, killed in a hit-and-run five years ago. When someone jumps from a skyscraper onto a truck, Bowden is sent to investigate. Meanwhile, five strangers are trapped in an elevator in the building where the jumper committed suicide. The communication radio in the elevator is broken but the guards, Lustig and Ramirez, observe the individuals via CCTV as events unfold. Tensions run high among those trapped, so Lustig calls the police and Detective Bowden assumes the case. Without being able to contact the individuals, he tries to work out who they are, but he can only account for four of them. Time is running out for the occupants of the elevator, as Bowden realizes he has to get them out quickly.

In 2009, 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 rapidly increase. Arriving at a party in Washington D.C., Helmsley presents his info to White House Chief of Staff Carl Anheuser who takes him to meet the President. In 2010, U.S. President Thomas Wilson and other international leaders begin a secret project to ensure humanity's survival. More than 400,000 people are chosen to board enormous "arks" constructed at Cho Ming, Tibet, in the southwest Chinese Himalayas. A Buddhist monk named Nima is evacuated while his brother Tenzin joins the Ark project. Additional funding is raised by selling tickets for €1 billion per person. By 2011, valuable items are moved to the arks with the help of art expert and First Daughter Dr. Laura Wilson. In 2012, Jackson Curtis is a science fiction writer in Los Angeles who works as a limousine driver for Russian billionaire Yuri Karpov. Jackson's ex-wife Kate, and their children Noah and Lilly, live with Kate's boyfriend, plastic surgeon Gordon Silberman. Jackson takes Noah and Lilly camping in Yellowstone National Park. After an encounter with Helmsley, they meet Charlie Frost, who hosts a radio show from the park. Charlie plays a video of Charles Hapgood's theory that polar shifts and the Mesoamerican Long Count Calendar predict that the 2012 phenomenon will occur. Charlie also tells Jackson that those who discovered this and plotted to inform the public were killed in various accidents. After an earthquake hits Los Angeles, the family returns home. Jackson grows suspicious and rents a Cessna 340 to rescue his family. He collects his family and Gordon as Earth crust displacement begins with a magnitude 10.9 earthquake, and they narrowly escape Los Angeles as California collapses into the Pacific Ocean. As millions of people die in earthquakes worldwide, the group flies to Yellowstone to retrieve a map from Charlie, escaping as the Yellowstone Caldera erupts. Charlie stays behind to broadcast the eruption and is killed. Learning that the arks are in China, the group lands in Las Vegas to find a larger plane. They find Yuri, his twin sons Alec and Oleg, girlfriend Tamara and pilot Sasha. Helmsley, Anheuser, and Laura Wilson are aboard Air Force One where they are heading to the arks. President Wilson remains in Washington D.C. to address the nation one last time. With the Vice President dead and the Speaker of the House missing, Anheuser appoints himself Acting Commander in Chief even though he is not in the presidential line of succession. Back in Las Vegas, the group steals an Antonov An-500 aircraft and they depart for China as the Yellowstone ash cloud engulfs the city. President Wilson is later killed by a megatsunami that sends the aircraft carrier USS John F. Kennedy crashing into the White House. When the group arrive in China, the plane runs out of fuel and crashes into a glacier, killing Sasha, while the others make a narrow escape. They are then spotted by helicopters from the Chinese army. Yuri and his sons, possessing tickets, are taken to the arks, leaving the Curtis family and Gordon behind along with Tamara because Yuri knows she had an affair with Sasha. The remaining group are picked up by Nima and taken to the arks with her grandparents. They stow away on Ark 4, where the United States contingent is aboard, with the help of Tenzin. As a megatsunami approaches the site, an impact driver becomes lodged in the gears of the ark's hydraulic doors, preventing a boarding gate from closing, which in turn prevents the ship from starting its engines. In the ensuing chaos, Yuri, Gordon and Tamara are killed, and Ark 4 is set adrift. Jackson and Noah dislodge the impact driver and the crew regains control of the ark before it collides with Mount Everest. Jackson rejoins with his family, and he and Kate reconcile. As flood waters from the tsunamis slowly recede, the arks travel to the Cape of Good Hope in South Africa. Helmsley starts a relationship with Laura.?