An earthquake ravages the city of Los Angeles, prompting Mike Roark, the determined head of the city's Office of Emergency Management, portrayed by Tommy Lee Jones, to abandon his vacation with his daughter Kelly in order to tackle the emergency. Although his associate Emmit Reese observes that the quake resulted in minimal damage, a tragic incident occurs when seven utility workers perish in a storm drain at MacArthur Park. Despite the city officials claiming that the incident was simply an accident unrelated to the earthquake, Roark is skeptical and attempts to suspend the subway operations running parallel to the affected area as a precaution.
In an effort to investigate, Roark defies regulations alongside his colleague Gator Harris, facing imminent danger when they are nearly engulfed by a surge of hot gases in the storm sewer. Meanwhile, geologist Dr. Amy Barnes, portrayed by Anne Heche, expresses concerns about a potential volcanic threat developing beneath the city, yet struggles to produce sufficient evidence for Roark to act decisively.
Early one morning, Dr. Barnes, alongside her assistant Rachel, returns to the site of the deadly incident to conduct further examinations. During their descent into the storm sewer, they uncover a fissure that previously emitted the dangerous gases. However, chaos ensues when another massive earthquake strikes, leading to Rachel's tragic demise as she falls into the crack.
As the situation escalates, smoke and lava erupt near the La Brea Tar Pits, with explosions sending steel grates skyrocketing into the air and causing a subway train packed with passengers to derail underground. Witnessing this disaster unfold from their vehicle, Roark and his daughter become distressed when she sustains injuries and is admitted to Cedars-Sinai Medical Center, under the care of Dr. Jaye Calder.
In a race against time, Roark and Dr. Barnes devise a critical plan to construct a barrier at Wilshire Boulevard and Fairfax Avenue aimed at containing the advancing lava flow. Thanks to their efforts, helicopters dump substantial amounts of water onto the molten lava, creating a protective crust. Although the operation yields initial success, Dr. Barnes continues to analyze geological data and predicts that the magma is still advancing underneath one of the subway tunnels.
The focus intensifies as they identify that a primary eruption will likely occur at the Beverly Center, perilously close to Cedars-Sinai. Compounding the urgency, Olber leads an underground rescue mission following the subway derailment, successfully saving all aboard, but tragically loses his life while trying to rescue the conductor from an encroaching lava.
Under Roark's guidance, the authorities utilize explosives to forge channels in the streets to redirect the flowing lava towards Ballona Creek, which will ultimately lead it to the Pacific Ocean. However, as Dr. Barnes analyzes the geological maps, she realizes that the street's incline is leading in the wrong direction. In response to this revelation, Roark orchestrates a bold strategy to demolish a construction site to block the lava's advance into the city.
When Gator makes the ultimate sacrifice to save a fellow officer trapped in the wreckage, their final explosive charge triggers a successful diversion of the lava, allowing it to flow safely toward the ocean, averting catastrophe in Los Angeles.
