Architect Doug Roberts returns to San Francisco for the dedication of The Glass Tower, a groundbreaking skyscraper he designed for developer James Duncan. Standing at an impressive 1,688 feet (515 m) and boasting 138 stories, it proudly holds the title of the world's tallest building. However, following an unfortunate electrical short-circuit that ignites a fire on the 81st floor during initial testing, concerns arise. Roberts discovers that Roger Simmons, the electrical subcontractor and Duncan's son-in-law, has compromised the tower's wiring. When confronted, Simmons pretends to know nothing about it.
As the dedication ceremony kicks off, public relations chief Dan Bigelow activates the tower's lighting, but in a bid to minimize electrical load, Roberts calls for them to be turned off. Suddenly, smoke plumes from the 81st floor, prompting the involvement of the San Francisco Fire Department. Joined by engineer Will Giddings, Roberts ventures to the afflicted floor, where tragedy strikes as Giddings heroically pushes a guard to safety, only to be fatally scorched by the frenzy of flames.
While the dedication celebration continues in the Promenade Room on the 135th floor, Roberts urgently informs Duncan of the fire. However, Duncan, who is busy courting Senator Gary Parker for an urban renewal contract, refuses to initiate an evacuation. Under pressure, SFFD Chief Michael O'Hallorhan coaxes Duncan to evacuate the Promenade Room’s guests. Simmons, at last, admits to Duncan that he skimped on safety measures to keep the project under budget, hinting that other subcontractors may have done so as well.
As flames engulf the express elevators, they claim the lives of several individuals whose elevator halts on the burning floor. Tragedy strikes again when Bigelow and his girlfriend Lorrie fall victim to flames that trap them in Duncan Enterprises' offices on the 65th floor. Meanwhile, Lisolette Mueller, a guest in the tower and target of con man Harlee Claiborne, rushes to the 87th floor to check on a deaf mother and her children. Security chief Jernigan manages to save the family but. Soon after, a gas line explosion demolishes the stairwell, isolating Roberts and the others from further escape.
Navigating through the destruction, they reach a service elevator that leads them to the 134th floor, only to find the entrance to the Promenade Room blocked by solidified cement. Determined to rescue those trapped inside, Roberts ingeniously accesses the room through a ventilation shaft while Lisolette and the children remain behind.
As firefighters work to control the blaze on floor 65, the electrical system fails and the passenger elevators shut down, prompting O'Hallorhan to rappel down the elevator shaft for safety. Just as firemen attempt to open the blocked Promenade Room door, another explosion collapses part of the last usable stairwell, sealing off the possibility of escape for those on the upper levels. After a harrowing wait, the firefighters manage to free the door, reuniting Lisolette and the children with Roberts and the others. In a desperate attempt to flee, Simmons tries to escape down the stairwell but is thwarted by the flames.
Amidst chaos, a helicopter rescue goes awry when two women recklessly rush the aircraft, causing it to crash on the roof and ignite flames. A Navy rescue squad quickly intervenes, attaching a breeches buoy between the Promenade Room and the roof of the adjacent Peerless Building, allowing some guests, including Patty Simmons, to escape. In a remarkable act of bravery, Roberts devises a "gravity brake" mechanism for the scenic elevator, enabling one trip down for a group of 12 people, among them his fiancée Susan Franklin, Lisolette, and the children. However, tragedy strikes as an explosion on the 110th floor sends Lisolette plummeting to her demise, leaving the elevator precariously hanging.
Just as fire swells toward the Promenade Room, Simmons leads a frantic group in a bid to commandeer the breeches buoy, but their attempt ends catastrophically as an explosion claims Simmons, Senator Parker, and others. In a courageous final measure, O'Hallorhan and Roberts opt to detonate water tanks atop the Tower with explosives, allowing a deluge to cascade through the structure and successfully douse the flames, leading to the survival of most remaining partygoers.
In the aftermath, a devastated Claiborne grapples with the reality of Lisolette's death, ultimately receiving her cat from Jernigan, while Duncan consoles his distraught daughter, assuring her that such a catastrophe will never recur. Roberts gratefully accepts O'Hallorhan's offer to guide him in the creation of a fire-safe skyscraper, as O'Hallorhan drives away, wearied yet resolute in the face of a redefined future.
