Shelby Gilmore (William Holden), who owns a newly constructed hotel on a remote Pacific island, wants desperately to marry his secretary, Kay Kirby (Jacqueline Bisset) and proposes to her under the impression that she’ll become his seventh wife. Kay is in love with Hank Anderson (Paul Newman), an oil rigger whose scientists are warning him that a nearby active volcano is about to erupt.
Shelby’s partner, Bob Spangler (James Franciscus), assures guests at the hotel that the threat of the volcano is a total exaggeration, explaining that it only erupts once every thousand years. Spangler is married to Shelby’s goddaughter Nikki (Veronica Hamel), but is cheating on her with Iolani (Barbara Carrera), an executive with Shelby’s hotel. Iolani is engaged to Brian (Edward Albert), the hotel’s general manager. Unbeknownst to all except Spangler, who chooses not to reveal the secret, Brian is his illegitimate younger half-brother and therefore part-owner of the resort.
Guests at the hotel include a bonds smuggler, Francis Fendly (Red Buttons), who is being tailed by a New York City private investigator, Tom Conti (Ernest Borgnine). Also on hand are Rene and Rose Valdez (Burgess Meredith and Valentina Cortese), who are retired circus tightrope walkers. Hank’s oil-rig workers include Tiny Baker (Alex Karras), who has a wager going with cockfighting rival Sam (Pat Morita) with a prized rooster that has just been delivered to him. Sam and his wife Mona (Sheila Allen) own a local bar.
Hank and Kay go for a picnic on the beach to discuss their relationship. During their time together, the volcano erupts and most of the island’s population are wiped out. Tiny and all of Hank’s workers are killed in town when a tidal wave crashes onto them. Sam takes Mona and two of his girls, Delores and Marsha, and escapes by car, while Hank and Kay rescue Nikki and some others at the stables by helicopter. The only survivors are those at Shelby’s hotel, overlooking a disaster that will surely come straight for them, as the volcano is spewing fireballs. A fireball lands at the hotel – Conti is blinded in the explosion, to Fendly’s horror. Conti is told by the paramedics that the injury is superficial and given time to heal, he will regain his eyesight. Some of the hotel guests panic and try to escape by stealing the helicopter, but it soon crashes, killing all those inside.
Hank insists that everyone must evacuate the hotel and journey to a safe side of the island to await rescue. Spangler convinces the majority of the guests to stay, including mistress Iolani. Shelby bids a farewell to Nikki, who insists on staying with her husband. After one final attempt to persuade others to join them, Hank and Kay leave the hotel along with Shelby, Brian, Rene and Rose, Fendly and Conti, Sam, Mona and the girls, plus a few more. At the hotel, Nikki stumbles upon her husband’s affair with Iolani, but now it is too late for her to follow the others though Spangler expels her from the hotel.
Trucks carrying survivors manage to travel as far as a mountainside gorge. Everyone must cross the gorge on foot. Conti is guided by Fendly, and the two become friends. From there, the party comes upon a rickety wooden bridge over a river of molten lava. Hank crosses first to see if it is safe. The others go in pairs. Two native children, whose father died crossing the gorge, are afraid and run away. Rose, who had stopped to rest with Rene because of her weak heart, dies after telling her husband to find the children.
After an explosion beneath the bridge causes Sam and Marsha to fall to their deaths into the lava, Rene hoists a child onto his back and recreates his old tightrope act in getting the child safely across. Hank guides the second child to safety. The survivors take refuge in a cavern, during which time fireballs streak across the sky. Spangler pays for his arrogance when one huge fireball arcs directly towards the hotel; when it hits, the hotel explodes, killing him, Nikki, Iolani and all who had unfortunately chosen to stay. The next morning, the survivors, fearing they’re the only ones left, continue on toward a beach to wait for rescue.