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Former pro-footballer Stewart Graff (Heston) is jogging near the Hollywood Sign. Back home, his wife Remy Royce-Graff (Gardner), bitter because of their failing marriage, starts her morning by picking a fight with him. After he has showered and is preparing to leave, Stewart finds Remy unconscious with a bottle of pills nearby. Accustomed to her periodic suicide attempts, Stewart prepares to induce vomiting when a violent tremor shakes the bedroom. Remy bolts out of bed, revealing that she was faking.
Meanwhile, at the Mulholland Dam, two workers begin a routine inspection of the structure in response to the tremor. One of them drowns.
In downtown Hollywood, LAPD Sgt. Lou Slade (Kennedy) and his partner Emilio Chavez (Pedro ArmendÃ¡riz, Jr.) are in pursuit of a suspect. They chase him out of their jurisdiction, finally crashing into the hedges at Zsa Zsa Gabor's house. After an L.A. County sheriff's deputy yells at Lou for ruining Gabor's hedges, Lou punches him out. He receives a temporary suspension.
On his way to work, Stewart visits Denise Marshall (Bujold), an actress who is the widow of one of his friends and co-workers. He drops off an autographed football for her son Corry (Tiger Williams) and helps Denise run her lines for a scene she is shooting later that day.
A junior staffer, Walter Russell (Niven), at the California Seismological Institute has calculated that Los Angeles will suffer a major earthquake in the next day or two. He frantically tries to reach his superior, Dr. Frank Adams. Another tremor hits as Adams and his assistant are working in a deep trench and they are buried alive. The scientists at the center argue about whether or not to go public with their prediction of a major quake. The acting supervisor, Dr. Willis Stockle (Sullivan), insists that if they are wrong their funding will be jeopardized. They agree on a compromise to alert the National Guard and police so that they can at least mobilize to help deal with the fallout.
While checking out at a grocery store, Rosa Amici (Principal) realizes she does not have enough money to pay for all her items. Jody Joad (Gortner), the seemingly kind store manager, insists that she keep the items and make up the deficit the next time she shops. Later, when he hears that the National Guard reserves are being called up on the radio, Jody leaves the store in the middle of his shift, revealing himself to be part of the reserve army. He goes to his run-down boarding house and changes into his NCO uniform - as well as a wig that covers his long hair - enduring the harassment of his housemates who, after seeing his bodybuilder wall art, accuse him of being homosexual.
Blowing off steam at a bar, Lou orders a double bourbon. A drunk (Matthau) periodically wakes up to toast random people. Daredevil motorcyclist Miles Quade (Roundtree) arrives with his partner Sal Amici (Dell) and his sister Rosa. A pool shark collects $50 that Miles owes him, leaving him short for the propane tank he needs to do a stunt for a talent scout. Miles convinces Lou to loan him the money.
The tremor cancelled Denise's shoot; she heads to Stewart's office, pretending to meet with a friend. When she runs into him, he is headed out for a drink. They go back to Denise's house for the drink and end up making love. He promises to come back later that night and invites her and Corry to spend the summer with him in Oregon, where he will be overseeing a project. Back at the office, his boss and father in-law Sam Royce (Greene) offers to hand over the company presidency to Stewart. He asks for some time to consider the idea. Stewart calls Denise and breaks off their plans for later that night. He goes to Sam's office to accept his offer but is stunned to see Remy there. He assumes she has convinced her father, Sam, to offer the promotion to Stewart in order to save their marriage. He storms out and she follows him, accusing him of cheating with Denise. He explains that he never had until that day.
When the couple get outside the building, the main earthquake occurs, which reaches 9.9 on the Richter Scale, destroying much of L.A. and killing thousands. All of the buildings are mostly damaged.
After the quake is over, Sam and most of his employees find themselves trapped on the upper floors of their 30-story skyscraper, as the elevators have crashed to the ground. They descend most of the way by the stairs, but the earthquake has collapsed part of the stairwell, leaving a gap of about two stories. Sam rigs a fire hose to a chair and lowers his staff down one at a time. When everyone is safe, he suffers a heart attack and Stewart climbs up to lower him down in the chair.
Denise's son, meanwhile, has been caught on a bridge over a spillway, which has become entangled with high voltage electric cables from a fallen transmission tower. She spies him unconscious on the concrete and climbs down to save him. Unable to climb back out with her son, she hails Miles and Sal who are passing by in their truck. After saving Denise and her son, they drive in search of help, coming across Lou who is organizing rescue efforts in the street. Lou commandeers their truck to use it as an ambulance.
Rosa is arrested by the National Guard for looting. Jody, now a sergeant, picks her out of a group of prisoners. Rosa assumes he is going to let her go, but he claims that the streets are too unsafe for her to go anywhere and he orders her to stay inside a secluded store. Another group of troops arrive with Jody's housemates as prisoners. Jody executes them in an act of revenge for all the ridicule he has endured from them, terrifying Rosa and his subordinates as they now see his dark psychopathic side.
Once Stewart gets everyone safely from his office to a shopping center named Wilson Plaza, now converted into a triage center, he goes off in search of Denise and her son. Sam is taken to Dr. Jim Vance (Nolan), but he does not survive. Stewart ends up driving Lou around in search of survivors and they come across Jody and his regiment. Jody threatens to fire on them if they come any closer and Rosa emerges when she sees Lou, Screaming and begging for his help. Lou and Stewart pretend to comply with Jody by driving away down the street and stopping out of sight. Meanwhile Sgt. Jody's scared men then leave to look for a superior officer to have Jody relieved from Duty, face a court martial and detainment. But Lou sneaks back and gets the jump on Jody, who's enraged at Rosa for betraying and rejecting him and then attempts to rape her. Lou then shoots Jody dead in self-defense and rescues Rosa.




The simulated collapse of Mulholland Dam â and the subsequent deluge â forms the film's climax.
As they drive away, they hear that another aftershock has destroyed Wilson Plaza. After a quick survey of the building, Stewart realizes there are survivors trapped in an underground garage three stories below ground. He and Lou crawl into the sewer and through some rubble with a jackhammer, which they use to drill through to the garage. Stewart is overjoyed to find Denise, who is one of the people trapped inside. As he hugs her, he sees his wife standing just behind her.
During the rescue, the Mulholland Dam finally gives way, sending water down the sewers. Lou and Denise make it up the ladder to safety, but as Remy climbs out, a man steps on the rung she's holding onto, injuring her hand in the process. She falls back into the water which is sweeping people away. Stewart looks up at Denise, but he cannot bring himself to abandon his wife to death. He sacrifices himself when he swims after her and both of them are swept away, along with others. Denise walks away from the manhole in shock and grief. Lou is holding Rosa when Dr. Vance walks up to him and says, "This used to be a helluva town, officer." Lou mumbles, "yeah."
Who do the simulated collapse of the Mulholland Dam and the subsequent deluge form?
film's climax