In an unnamed U.S. city where politics run dirty, the crooked Mayor William Dudley [Leslie Nielsen] approves a big oil refinery right in the town center, far from any river, lake, or reservoir. On a sweltering summer day, Herman Stover [Jonathan Welsh], a dangerously unsettled employee at the refinery, is denied a hoped-for promotion and then fired after resisting a departmental transfer. Consumed by a petty urge for revenge, he opens the valves to the storage vats and the interconnected pipes, flooding the streets with gasoline and chemicals. It doesn’t take long for this act of vandalism to ignite a fire, a cascade of explosions at the refinery, and a mushroom cloud of flame that begins to consume the city.
The disaster shifts its focus to a newly built hospital—part of the mayor’s ambitious, but poorly planned civic program—that stands as a symbol of brisk urban growth yet remains shoddily constructed and ill-equipped to handle a mass casualty situation. There, head doctor Frank Whitman [Barry Newman] and his staff, including nurse Andrea Harper [Shelley Winters], work around the clock to treat thousands of injured residents. In the background, the city’s reporter Maggie Grayson [Ava Gardner], another symbol of the era’s ambition, sees the unfolding crisis as a rare chance to propel her career onto a national stage. Quietly entwined with the hospital storyline is Diana Brockhurst-Lautrec [Susan Clark], a wealthy socialite and widow of the governor after whom the hospital is named, who is secretly involved with the Mayor to push her social standing higher. The tension intensifies as Herman Stover arrives at the hospital during the dedication ceremony, having known Diana since high school and lingering with a dangerous fixation.
No one discovers that Stover is the true source of the citywide fire, and his mind remains locked in a fog of confusion and guilt—he cannot fully grasp or accept what he has done. When the hospital becomes ringed by the encroaching flames, Chief Risley [Henry Fonda] orders his son, Captain Harrison Risley [Richard Donat], and the fire crews to implement a daring plan: a “water tunnel,” a channel of hoses laid across a burning street to reach and rescue the hospital. The operation preserves the building and saves many lives, but not without cost; casualties among hospital staff and patients mount, and Stover and Harper both become casualties in the chaotic moment—Stover falls to debris and Harper perishes in a bid to rescue him. Diana, the Mayor, and Dr. Whitman manage to escape the hospital as the flames engulf the building.
The next day shifts to a quarry outside the city, where a makeshift camp has sprung up to house the thousands left homeless by the disaster. There, Whitman and Diana acknowledge their love, while Mayor Dudley takes a victory lap, telling reporters that the real heroes are the people of the city. Maggie Grayson, still reporting from the studio, signs off her broadcast and heads out on a date with Jimbo [James Franciscus], her loyal ally through the crisis. As the city reels from the destruction, Chief Risley’s final message lands with a sobering echo: it takes only one man to destroy a city.
