When a massive earthquake rocks the city of Los Angeles, Emergency Management department head Mike Roark (Tommy Lee Jones) returns from his vacation to help with the city's response. After geologist Dr. Amy Barnes (Anne Heche) warns that a volcano may be forming in sewer tunnels, another severe earthquake unleashes the lava flowing underfoot, threatening to destroy the whole city. As the fiery molten rock runs through the streets, Roark and Barnes must figure out how to divert it.