Doug Roberts (Paul Newman) is chief architect for Duncan Enterprises, an architectural firm specializing in skyscrappers. Their greatest project, the Glass Tower, is 1,800 feet high and set for dedication in San Francisco, and a lavish ceremony is scheduled to include Mayor Robert Ramsey (Jack Collins) and Senator Gary Parker (Robert Vaughn). Jim Duncan ( William Holden), the company's president, is wooing Senator Parker because his support for an urban renewal effort will allow the company to build more such skyscrappers. When Roberts flies back to San Francisco for the party, he finds in his office his fiance, magazine editor Susan Franklin (Faye Dunaway), and when the two renew romantic acquaintances, she reveals that she has received a long-sought promotion - which throws a wrench into their plans to move to the countryside to have a family.

Doug, however, is interrupted when he gets a call from the Tower utility room in the building's massive basement. In routine checking of the building's generators, a power surge blows out a circuit breaker - and unknown to everyone the surge blows out another circuit breaker, this one on the 81st floor in a large storage closet that sends a wire flopping onto shop rags that catch fire.

Roberts consults his pal, Will Giddings (Normann Burton), and the two confront Jim Duncan about the wiring; Duncan's chief electrical engineer is his son-in-law, Roger Simmons (Richard Chamberlain), and to all three men's chagrin Simmons cannot be reached by phone. While Will Giddings works with the building's chief security officer, Harry Jernigan (O.J. Simpson), on the electrical systems, Doug Roberts tracks down Roger Simmons and his wife Patty (Susan Blakely), and confronts Simmons about the wiring. Simmons, a thoroughly oily man, insists the wiring is up to city safety codes, but those codes are not sufficient for an 1,800-foot skycrapper. Simmons nonetheless blows off Doug, which agitates Patty as she doesn't want her father's business to suffer because of him.

The dedication ceremony goes off as scheduled and the entire building is lit up for the party in the Promenade Room some 135 floors up. Doug and Will miss the party as they examine more wiring, and find it to be in worse shape than they could have imagined. Roberts gruffly orders the main utility room to shut off the power overload (in the process overruling Jim Duncan), then he and Will head to the 81st floor to check relay systems - just as security cameras detect smoke from that floor's storage closet and Harry Jernigan has the fire department called. He proceeds to 81, but before he arrives Doug, Will, and other executives spot the smoke and the fire erupts into the hallway, engulfing Will Giddings. Doug and the others smother Giddings in drapes while one man grabs a fire hose and begins spraying the flaming storage closet. Jernigan arrives and orders Doug to call an ambulance and also to call Jim Duncan and evacuate the Promenade Room while he helps get Giddings to the ground floor. But when Doug calls Duncan, Duncan insists the fire won't spread, and insists that Doug attend the ceremony.

The fire department now arrives in force with Battalion Chief Mike O'Halloran (Steve McQueen), who has a distaste for architects because they insist on building skyscrappers even though the department can't easily fight a fire above seven floors. Despite this antagonism, Roberts and O'Halloran work together as the department sets up a forward command two floors below the fire. O'Halloran quietly goes to the Promenade Room and confronts Jim Duncan to evacuate the area, and Duncan reluctantly proceeds. But a gas line ruptures and explodes, detonating more fires around the building and reaching the main elevator banks - and catching a crowd of partiers taking the elevator down only for the elevator to stop on the fire floor and the blaze to engulf them.

One of the partiers, painter Lisolette Mueller (Jennifer Jones), being entertained by shady businessman Harlee Claiborne (Fred Astaire), doesn't evacuate the building; she instead rushes to the 87th floor where a deaf woman, Mrs. Allbright (Carol McEvoy), lives with her two children. When security cameras spot Mueller, Harry Jernigan and Doug rush to the 87th floor to rescue the Allbrights. Jernigan rescues Mrs. Allbright, but the fire cuts off her two children and Lisolette Mueller, and when a gas line explodes on the stairway, Doug, Mueller, and the Allbright children must trek up 47 floors to the Promenade Room, in the process surviving a gas explosion that shatters the stairway. They eventually arrive on the 135th floor but now must find a way through a fire door sealed shut by construction cement left over from the week.

When word of the sealed door reaches the fire department, two firefighters laborously trek up to the 135th floor and succeed in blowing open the door, but the stairway below has been destroyed, and with the power failing, the best hope to evacuate the Promenade Room now is helicopter landing on the rooftop helipad - which disintegrates when several panicking partiers rush to the chopper and it has to swerve out of the way and promptly crashes. Now the only hope is to set up a Breecher's Buoy from a building a block away. Doug, however, activates a gravity brake on the outside elevator, so twelve people - the women who were to be carried out on the helicopter, plus the Allbright children, Lisolette Mueller, and one of the two firemen - can at least get to the ground. This, however, goes wrong when more explosions tear the elevator off its track and throws Lisolette Mueller into a fatal fall, forcing O'Halloran to lead a laborous effort with a helicopter to wrench the elevator from its track and place it on the ground with the survivors.

After hours of work, the fire reaches critical mass, and O'Halloran is given the grim truth - the only hope left is to explode the building's water tanks on the roof, as they hold one million gallons of water and blowing the tanks is the last hope of extinguishing the fire. This option, however, may kill the remaining partiers trapped on the Tower, but with no other choice O'Halloran and Roberts set to work planting explosive charges, then secure themselves with the remaining partiers in the Promenade Room for the fateful vigil before the explosion and ensuing flood.