In Los Angeles, engineer Stewart Graff returns home from his morning jog to have another petty quarrel with his wife Remy about his lack of attention toward her. A little later, Stewart steps out of the shower to find Remy passed out, apparently from an overdose of pills, an action she has taken several times before. A mild temblor reveals that Remy is feigning, however, as the shaking sends her leaping to her feet in terror. Despite Remy’s theatrical response to the temblor Stewart, discouraged by their constant bickering, leaves for work, but stops at the modest Hollywood cottage of the recently widowed, aspiring actress Denise Marshall to deliver a football to her young son Corry. That morning, at the Hollywood Reservoir, technicians Red and Max begin an inspection of the dam, a mandatory requirement after the minor quake. On the streets of Hollywood, patrolman Lew Slade and his partner, Emilio Chavez, ignore the mild quake as they chase a thief driving a stolen sports car, forcing the speeding thief into the dense hedge of a lavish home. As the thief escapes, Slade turns on the sheriff who has joined the pursuit, and when the officer upbraids Slade for infringing upon the sheriff’s turf, the patrolman angrily punches him in the face. At the California Seismological Institute, graduate assistant Walter Russell, unable to contact his mentor Dr. Adams, tentatively reveals to Institute’s head, Dr. Stockle his statistical theory that the morning’s temblor will be followed by another, and then a massive quake within twenty-four hours. In the San Fernando Valley, stunt motorcyclist Miles Quade and his manager, Sal Amici, prepare for Miles’s tryout performance for the manager of a lavish Las Vegas hotel. Nearby, Sal’s sister Rosa shops at a local convenience store under the admiring eye of clerk Jody. Meanwhile, Slade, who is despondent after being placed on suspension for striking a brother officer, visits a local bar, where he announces he is through with police work. At the dam, Max and the chief inspector note with alarm that the water level has risen an inch in one hour. Within the following hour, as Walt has predicted, another mild quake strikes the region. When Walt insists that the public should be told of his prediction, Stockle reminds him that the information would likely set off a panic. That afternoon, while departing his office at Royce, Inc., where he works for Remy’s father, Sam Royce, Stewart runs into Denise, who agrees to go to lunch with him. Giving way to his attraction to Denise, Stewart accompanies her to her home, where they have sex. Afterward, Stewart reveals a new project he will be starting soon in Oregon and asks Denise to accompany him there. Touched, Denise agrees to consider the offer. Unnerved by Walt’s report, Stockle meets with Mayor Lewis to recommend that certain city-wide alerts be implemented. Learning that several National Guard units have been ordered to report for duty, Jody leaves the convenience store to join his unit, but stops by Rosa’s apartment where he peeps through the windows and secretly watches her changing clothes. Minutes later, Jody knocks on Rosa’s door and offers to inspect her apartment for damage from the quakes. Uneasy with Jody’s overbearing manner, Rosa departs as soon as possible to see a movie. Returning to the office, Stewart is startled when Sam offers to make him company president. Then when Sam acknowledges this will mean canceling the Oregon project, Stewart hesitates. Deciding moments later to accept the offer, Stewart is surprised when Remy steps out of her father’s private office. Realizing that Remy, who has harbored suspicions about Stewart and Denise, has pressured Sam into making the promotion to interfere with the burgeoning romance, Stewart angrily storms out of the office. In Hollywood, after Corry drives away on his bicycle, Denise takes a walk in a canyon nearby while Miles and Sal greet the Las Vegas representative. At Los Angeles International Airport, as an east coast flight makes its landing approach, a massive quake lasting nearly a minute strikes Los Angeles. Throughout the city, the intense shaking causes buildings to sway violently, then collapse, showering debris onto pedestrians and traffic in the streets. On freeways, drivers lose control of their vehicles and careen off ramps. As the runways at the airport split open, a traffic controller frantically informs the landing jet, enabling the pilots to get back into the air without damage. At the same time, in the theater, Rosa and the other panicked movie goers rush outside as the building crumbles around them. In another part of the city, Denise staggers across a hillside, barely managing to avoid a house tumbling down toward her. Nearby, Corry, cycling across a small bridge over the dry Los Angeles riverbed, tumbles into the concrete channel. Arguing outside in front of the Royce, Inc. building as the quake hits, Stewart and Remy duck underneath his large sports utility vehicle. At police headquarters, large pipes fall onto Chavez, while at the Seismic Institute the equipment is jarred loose due to the quake’s violence. When the shaking stops, much of the city lies in ruins. At Royce, Inc., Sam takes command, moving injured staff away from the gas oozing from cracked pipes. After the elevator collapses from being overloaded, many workers begin hurrying down the stairs, only to find that the stairwell ends abruptly into the open air. Assembling a chair tied to the fire hose, Sam and the others lower surviving workers one by one down several floors to Stewart, who has climbed up to the eighteenth floor to help. Unable to find Corry at home, Denise searches for him among the collapsed houses and eventually spots him unconscious in the channel. Climbing down to Corry, who is surrounded by live electrical wires, Denise struggles to lift him up the hill. Calling for help, Denise is relieved when moments later Miles, whose stunt track has collapsed, drives by with Sal in their truck. The men rescue Denise and Corry, and a little later, meet up with Slade, who commandeers the truck to pick up more injured people. At the reservoir dam, Max joins the chief inspector and discovers several large cracks in the structure. After receiving their report, the mayor gives a speech over the radio advising the public to move to high ground as soon as possible. At Royce, Inc., Sam, who has over-strained himself, suffers a heart attack after lowering all his staff to safety, forcing Stewart to come to his aid. Stewart then drives Sam and others to a make-shift medical triage area set up in the remains of the nearby Wilson Center, before hurrying away to find Denise. Discovering that Rosa has been picked up by a fellow Guard member on suspicion of looting, Jody takes responsibility for her. Later, when Jody comes upon two of his neighbors who have also been accused of looting a pawn shop and who habitually ridicule him, Jody terrifies Rosa and others nearby by shooting the men. Arriving with Slade at the Wilson Center, Denise waits anxiously while Dr. Vance takes Corry to be examined and learns from a Royce, Inc. secretary that Stewart is safe. A little later, Dr. Vance pronounces Sam dead, but decides not to inform the emotionally volatile Remy. Meanwhile, Slade, having returned to the streets to assist, commandeers both Stewart and his large car to transport the injured. As a major aftershock rattles the city, causing further damage and death, Rosa struggles to get away from Jody, who has sequestered her down an alley where he attempts to force himself on her. Recognizing Slade as he drives by with Stewart, Rosa screams for help, and the policeman comes to her aid, shooting the maddened Jody. Returning to the Wilson Center in the hope of finding Denise, Stewart learns that the aftershock has sealed off the triage center, located in the sub-basement. Because he designed the building, Stewart knows its structure and convinces a city planner that he can reach the basement through storm drains to look for survivors. Stewart persuades Slade to join him and the men take emergency lighting and a jackhammer through the drains and eventually discover seventy survivors, including Denise, Corry, Remy and Dr. Vance. As Stewart embraces Denise, the reservoir dam bursts sending billions of gallons of water toward the ravaged city. The rescue from the triage center begins safely, but the water reaches the storm drains just as the last survivors are crawling up to safety. Among them are Stewart and Remy, who is almost up the ladder to the street when she slips and falls back into the waters. Staring up at the waiting Denise, Stewart hesitates then reluctantly turns back to rescue Remy, only to be swept away with her by the surging water. As the world learns of the destruction of Los Angeles, a young engineer scheduled to meet Stewart earlier that day, declares his determination to help rebuild the city.