The capsized luxury liner, S.S. Poseidon is still afloat after six survivors have been rescued via helicopter.Tugboat captain Mike Turner (Michael Caine) discovers the shipwreck. Accompanied by second mate Wilbur (Karl Malden) and passenger Celeste Whitman (Sally Field), he heads out to claim salvage rights, as the tugboat Jenny lost her cargo in the same tsunami that capsized the Poseidon.They are soon followed by Dr. Stefan Svevo (Telly Savalas) and his crew, who claim to be Greek Orthodox medics who received the ship's SOS. They board the doomed vessel through the bottom hull opening left by the Greek rescue team, and become trapped after the entrance collapses. The group with Turner also encounters the ship's nurse, Gina Rowe (Shirley Jones) and two passengers - Suzanne Constantine (Veronica Hamel), and war veteran Frank Mazzetti (Peter Boyle) who is searching for his missing daughter Theresa (Angela Cartwright), whom they soon find along with the elevator operator Larry Simpson (Mark Harmon) and as well as a "billionaire" named "Tex" (Slim Pickens). Later they find the blind Harold Meredith (Jack Warden) and his wife Hannah (Shirley Knight), who were waiting to be rescued.Water continues to submerge decks and more explosions occur. Turner and his group find the purser's office, where Svevo decides he and his men will continue their search for other survivors. Turner agrees, and Svevo and his men leave in another direction. Another explosion causes the safe in the purser's office to fall through the bulkhead and open, revealing gold coins (each worth 100 times its weight in gold) and cash. Turner and Wilbur excitedly gather it and start looking for another way out of the ship. Unknown to Turner and the survivors, Suzanne is actually working with Svevo and takes a list containing information about a cargo of crates from the purser's office. She leaves the group and goes to find Svevo. She gives the document to Svevo but decides she no longer wants to go through with his plan, preferring to go back and get out with Turner. Svevo apologizes and orders Doyle, one of his men, to kill Suzanne. He shoots her, but she strikes him with an axe, killing him, before she dies. While making their way up through the decks, Turner and the others find Suzanne's corpse and reach the unpleasant conclusion that a murderer is on board. During one climb towards another deck, Hannah dislocates her shoulder while helping her husband.After continuing their progress through the ship, Turner and his group find Svevo and his men gathering a cargo of plutonium. Svevo reveals that his real intention for boarding the Poseidon was to retrieve his lost shipment of plutonium, adding that he can't let Turner and his group go now. However, before they can kill them, another explosion occurs, allowing them to escape through another cargo room full of cars. Turner, Mazzetti and Simpson find some guns and attempt to make a fight of it - in the ensuing shoot-out, Mazzetti and another of Svevo's men are killed. Water floods the deck as Turner's group proceed up to the next deck, but an injured Hannah is unable to climb a ladder: she falls into the rising water and drowns. While trying to rescue her, Turner loses all of his salvaged gold. Svevo and his one remaining gunman head back up to the ship's stern, where the rest of Svevo's team attempt to use a crane to raise the plutonium up to the hull of the ship, which is still above water but is slowly sinking.In another section of the ship, Turner and the survivors exit the ship through an underwater side door, but due to shortage of scuba tanks, Wilbur (unknown to Turner and his group) sacrifices himself by swimming underwater and disappearing. Turner and Celeste swim to the tugboat Jenny and move it closer to the Poseidon as the remaining survivors swim towards it. Svevo's men see them and open fire, killing Tex, who willingly holds onto a bottle of champagne as he goes down. Turner's group makes it to his boat, and they sail away. Water continues to flood the Poseidon and cause the on-board boilers and then the plutonium cargo to explode. Svevo and his men are all killed as the entire Poseidon explodes.On board Turner's boat, Turner accepts that his tugboat Jenny will be taken from him when they get to port, but Celeste reveals a diamond she salvaged from the Poseidon. Celeste asks Turner, "Are you going to kiss me now?" and Turner replies, "I was going to kiss you anyway." They kiss and the tugboat Jenny sails away into the sunset with the survivors.
Output: Question: What causes the on-board boilers to explode?, Answer: Water flooding into the Poseidon.