Plot
The capsized luxury liner S.S. Poseidon is still perilously afloat after six survivors had been rescued by the French Coast Guard. While at sea, tugboat captain Michael "Mike" Turner, and his elderly first mate, Wilbur, spot the very rescue helicopter containing the Poseidon's original survivors, and after having seen the aircraft so far out at sea, they are compelled to believe that a large ship must be in precarious straights, so they elect to get a fix on the chopper's course in the hopes of ascertaining coordinates requisite enough to lead them to a prospective shipwreck's position. Accompanied by passenger Celeste Whitman, he heads out to claim salvage rights, as his tugboat, Jenny, which lost her cargo in a raging storm at sea, one which was a considerable distance from the tsunami-capsized S.S. Poseidon, freeing them of the same possible fate, the considerable salvage from a large ship would more than help make up for the cost of his lost cargo, and pay off the balance of his delinquent, long-overdue mortgage on his tugboat, one of which he can no longer afford.
They are soon followed by Dr. Stefan Svevo and his three-man crew, who claim to be Greek Orthodox medics who heard the ill-fated liner's SOS transmission and responded to help render rescue and medical attention, should there be survivors at the scene. Both parties board the vessel and gain entrance through its stern section, made possible through a small section cut from the hull by the French Coastguard, with this original exit for the previous film's characters, but is now a point of entry for this installment's principals. Turner and company then enter into the small utility compartment which is coupled with, and partially houses, the ship's propeller shaft. After emerging from the compartment, the collective party, at the very onset of their exiting that section, encounter raging fires, mangled catwalks, churning waters, and other such hazards, upon their discovery of the ship's engine room. They see, as well, the dead body of Linda Rogo, who fell to her death in The Poseidon Adventure . Upon seeing the conditions aboard, Wilbur then exclaims to Captain Turner that the ship is "a floating time bomb". However, Turner remains undeterred and maintains their carrying on further through the ship. After happening upon an access grate, Turner discovers a vertical ventilation shaft, and inside it, access points at different deck levels, connected by way of a secured ladder. After descending the shaft, Turner comes upon another access grate, and behind it, the ship's gym. After the group enters the room, they come across three survivors: two passengers, a tavern owner and World War II veteran named Frank Mazzetti, who has been desperately searching the liner for his missing daughter, Theresa; the beautiful and elegantly-dressed Suzanne Constantine; and a crewmember, Gina Rowe, a ship's nurse--all of whom had been trapped in the adjoining steam room, and are then rescued by Turner and Svevo. Theresa is found, as are a ship's elevator operator named Larry Simpson, and a "wealthy oilman", Dewey Hopkins, nicknamed "Tex", who clings to a rare and valuable over-100-year-old bottle of wine. The group also finds two more passengers hoping to be rescued: blind renowned novelist Harold Meredith, and his protective, lovingly-devoted wife, Hannah.
Water continues to submerge decks, and more explosions occur throughout the vessel. Turner's group find the ship's purser's office, where Svevo decides he and his men will search aft for other survivors, parting ways with the rest, who continue to move forward. Another explosion causes further shockwaves through the Poseidon. This explosive succession causes the large, wall-bolted safe in the purser's office to fall through the floor down (up) into the next deck. The impact of the safe's fall renders it disabled and now open, revealing a large cache of rare gold coins, tens of thousands of U.S. dollars, stock certificates, and a small steel box containing large, highly-valuable, industrial-grade Bort diamonds. Turner, Celeste and Wilbur then proceed to fill several bags with gold, cash, and gems from the now-broken-open safe.
Unknown to Turner and the survivors, Suzanne is actually working with Svevo. She takes a list containing information about a cargo of crates from the office. Going off on her own, she gives Svevo the document but decides to rejoin Turner's group. Svevo orders Doyle, one of his men, to kill Suzanne. After being shot, a dying Suzanne strikes Doyle with an axe, killing him. While making their way up through the decks, Turner and the others find Suzanne's corpse and realize a murderer is on board.
Hannah dislocates her shoulder while helping her husband. Svevo and his men are found 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 cannot let Turner and his group go now. However, before anyone is killed, another explosion occurs, allowing Turner's group to escape.
Turner, Frank, and Larry find guns. In the ensuing shoot-out, Frank and Castrop, another of Svevo's men, are killed. Water floods the room as Turner's group proceeds up to the next deck. An injured Hannah, unable to climb a ladder, falls into the rising water and drowns. While trying to rescue her, Turner loses all of his salvaged gold, cash, and diamonds. Svevo and his one remaining gunman head back up to the ship's stern, where the rest of Svevo's team attempt to retrieve the plutonium, firearms, and ammunition up to the hull, which is still 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 sacrifices himself by swimming underwater and disappearing. Turner and Celeste swim to the tugboat and move it closer to the Poseidon as the remaining survivors swim towards it. Svevo's men see them and open fire. Tex, who in reality was not a wealthy passenger but a sommelier (part of the Poseidon's crew), holds onto his wine bottle as he is gunned down and perishes. The rest of Turner's group makes it to the tugboat and sail away. Water continues to flood the Poseidon, causing the boilers and then the plutonium cargo to erupt, exploding the ship's hull and sinking it stern first. As it sinks, Svevo and his men are killed.
En route, Turner accepts that his tugboat will be taken from him when they reach port in Marseilles, but Celeste reveals a diamond she had hidden on her person from the now-lost steamer. The two kiss, and the tugboat sails away into the sunset, bringing the total survivors of the liner's disaster to ten, both passengers and crew.
