The film commences with Stewart Graff (Charlton Heston) engaged in a jogging session beneath the iconic Hollywood sign, setting a seemingly ordinary day in motion. Upon returning home, he concludes his workout on a resistance machine, only to be met by his contentious wife, Remy Royce-Graff (Ava Gardner), who begins to provoke a rather explosive argument. The intensity escalates when Stewart, with palpable frustration, exclaims to Remy, > "You'd hardly call this a marriage, wouldn't you?" This confrontation takes a chilling turn when, after showering and preparing to leave, he finds Remy sprawled unconscious with a bottle of pills at her side. Confronted with her ongoing battle against depression and her periodic suicide attempts, he instinctively prepares to induce vomiting. However, a sudden and violent tremor jolts the bedroom, prompting Remy to spring from the bed, revealing her deception.
As the seismic events unfold, we shift to the Mulholland Dam, where two workers are conducting their routine inspection in the wake of this tremor. Tragedy strikes when one of the workers becomes trapped in a flooded elevator shaft, leading to his untimely demise.
In the heart of downtown Los Angeles, Sgt. Lou Slade (George Kennedy) and his partner Emilio Chavez (Pedro Armendáriz, Jr.) are on the tail of a suspect. Their pursuit leads to an unexpected collision at Zsa Zsa Gabor's property where, in a fit of rage against a Beverly Hills Sheriff's reprimand regarding the damage to the hedges, Lou retaliates with a punch, illustrating the mounting tension and frustration within him. This anger stems from the callous nature of his job, exemplified by his superior's indifference after Lou reveals that a car thief caused grievous harm to a child by failing to stop after an accident. Consequently, he faces a temporary suspension from duty.
In a twist of fate, while making his way to work, Stewart pays a visit to Denise Marshall (Geneviève Bujold), an actress mourning the loss of her husband, who was one of Stewart's colleagues. In a compassionate gesture, he presents her son with an autographed football and assists her in rehearsing lines for a forthcoming scene, showcasing his caring nature amidst his turbulent marital life.
Meanwhile, a junior staffer named Walter Russell (Kip Niven) at the California Seismological Institute is grappling with a chilling realization: Los Angeles is on the brink of experiencing a catastrophic earthquake within the next two days. Desperate to communicate this urgent prediction to his superiors, he encounters a fatal turn when a tremor traps him and his boss, Dr. Frank Adams (Bob Cunningham), deep within a trench. Amidst rising panic, the scientists debate whether to alert the public, fearing the ramifications of a potential warning could spark more chaos than the quake itself. Ultimately, a compromise is struck to inform the National Guard and police, allowing them to prepare for the aftermath.
As the day progresses, we glimpse into the life of Rosa Amici (Victoria Principal), a financially struggling young woman at a grocery store who finds herself unable to cover her purchases. However, the seemingly kind store manager Jody Joad (Marjoe Gortner) allows her to take the items, only for his true intentions to unravel later on when he abandons his post to fulfill his responsibilities as part of the National Guard.
In a lively bar scene, Lou indulges in a drink while overlooking the illegalities unfolding around him. A chance meeting leads him to Miles Quade (Richard Roundtree), a daring motorcyclist looking to secure funds for a stunt, showcasing the interwoven fates of the characters as the tension in Los Angeles heightens.
Back at Denise's place, after she cancels her filming due to the tremors, she and Stewart connect over drinks, leading to a romantic entanglement filled with promise and tension. Amidst their growing bond, Stewart faces an unexpected offer from his boss, Sam Royce (Lorne Greene), to step into a leadership role within the company. Overcome by the chaos of his feelings for Denise and his fracturing marriage, Stewart navigates a turbulent emotional landscape leading to a tumultuous confrontation with Remy.
As the ground gives way, Los Angeles experiences a cataclysmic earthquake, the likes of which the world has rarely seen, culminating in catastrophic destruction and massive loss of life. The quake, with a staggering magnitude of 9.9 on the Richter scale, sends shockwaves through the city and through the lives of our protagonists.
In the aftermath of the disaster, chaos reigns as Sam and various surviving employees find themselves trapped in their skyscraper, enduring harrowing challenges as they attempt to escape. Sam displays extraordinary courage in aiding his colleagues when he rigs a fire hose to assist them down, only to succumb to a heart attack amidst their escape.
Denise's plight escalates as she finds her son unconscious on a freeway overpass during the quake. In a dramatic moment of desperation and heroism, she reaches out for help, leading to a harrowing rescue as Lou takes charge of the rescue operations on the street.
As the narrative unfolds, we delve deeper into the psychological ramifications of survival as Jody, now a sergeant, reveals his dark transformation, dispensing vigilante justice against former bullies amidst the chaos, further complicating the already dire situation.
Stewart and Lou embark on a journey through the wreckage, searching for Denise and her son while navigating the remnants of their shattered lives. Their paths lead to encounters that bring out not just physical survival instincts but moral dilemmas, decisions laden with emotional weight and human resilience.
The film races towards its poignant conclusion, marked by tragic sacrifices, the struggle for survival, and the overwhelming grief that follows catastrophic events. As the sun sets on a destroyed Los Angeles, Lou is left with poignant reflections on their once vibrant city, encapsulated in the heavy silence as he contemplates, "This used to be a helluva town, officer," leaving the audience to grapple with the profound impact of loss and the indomitable spirit of humanity amidst devastation.
