En route to North Africa across the Mediterranean Sea, Capt. Mike Turner, first mate Wilber and a spunky traveler named Celeste Whitman encounter a violent storm. Mike’s tugboat, Jenny, perseveres, but valuable cargo falls overboard, which means financial ruin for Mike’s business. The next morning, the sea is calm and Mike and Wilbur notice a helicopter from the French coast guard, indicating a ship is in distress nearby. Mike follows the helicopter’s path, in hope of a moneymaking salvage opportunity. Soon, he spots the overturned hull of the luxury cruise ship S. S. Poseidon, which capsized during the storm. Believing a fortune might still be locked inside the purser’s office, Mike wants to go aboard before the ship sinks completely. As explosions inside the hull shake the vessel, Wilbur and Celeste are dubious about the plan. Arriving on the scene in a swanky yacht is Dr. Stefan Svevo. When Mike tells Svevo he has already claimed salvage rights, Svevo assures him that he and his crew are only there to give medical aid to any remaining survivors. Mike, Wilbur and Celeste, along with Svevo and three of his crew, climb into the Poseidon through an opening in the hull cut by the coast guard. Inside, oil fires burn, and the group proceeds cautiously through the inverted engine room. From there, they crawl through an air shaft leading to the ship’s gymnasium and steam room, but a sudden tremor collapses the shaft behind them, blocking off their escape route. Hearing screams, they rescue three surviving passengers trapped in the steam room: Gina Rowe, the ship’s nurse; Frank Mazzetti, who is hysterical about his missing daughter; and Suzanne, who hides the fact that she knows the mysterious Svevo. After locating a deck plan, Mike remains determined to make his way to the purser’s office. Despite Mike's greedy motives, the group elects to stay together and follow him. Meanwhile, Svevo discreetly studies and keeps the deck plan for himself before taking his crewmen in the opposite direction. As the group arrives at the purser’s office, another tremor causes a locked safe to plunge through the floor to the adjacent deck. Upon impact, the door springs open, revealing gold coins, cash and jewelry. As Mike, Wilbur and Celeste fill bags with the treasure, three more survivors emerge: a drunken Texan named Dewey Hopkins, who Mike nicknames Tex; Theresa Mazzetti, Frank’s lost daughter; and Larry Simpson, a ship bellboy who saved Theresa’s life. While Frank and the others are distracted, Suzanne sneaks away to locate a cargo inventory list for Svevo. The group searches for a way out, but Wilbur lags behind, stricken by pain from a fatal disease. Mike reassures his first mate that they now have the money to pay for an operation. Along a corridor, the group picks up two additional survivors, Hannah Meredith and her husband Harold, who is blind. Unable to locate an exit, Mike decides they should rendezvous with Svevo, who kept the deck plan. On the way, they find Suzanne lying in a corridor, dead from a gunshot wound. Meanwhile, Svevo and his crew have been busy searching the cargo hold and have opened several crates of weapons and ammunition. Thanks to the inventory Suzanne found, Svevo knows that the crate he seeks is marked “Sprague Pumps,” but he killed Suzanne because she had knowledge that the crate actually contained plutonium. When Mike and the group reach the cargo hold, Svevo’s men emerge with their guns drawn. Pointing to the barrel of plutonium, Svevo announces that he must kill the group, now that they have discovered his true mission. A tremor rocks the ship, allowing Mike, Frank and Larry time to grab guns and fend off the terrorists. In the exchange of fire, Frank is shot dead. After Svevo traps the group, the compartment floods, but Harold’s acute hearing detects a banging door nearby through which the group is able to escape. On the other side of the door is a corridor filled with rising water, but leading to a ladder. As the last person to swim and climb the ladder, Hannah struggles and suffers a fatal fall. In an effort to save her, Mike loses his newly-found fortune. Next, Mike discovers an escape route, but it requires swimming through an underwater door using scuba gear. As Wilbur’s pain becomes unbearable, he discreetly swims away to his death. The others make their way to the surface. At the hull’s opening, Mike notices Svevo and his crew loading crates. He and Celeste swim underwater to retrieve the tugboat. When they crank Jenny’s engine, Svevo realizes that Mike and his group are still alive and orders his men to shoot. Celeste fires back while Larry, Theresa, Gina, and Harold swim safely towards the tugboat, but Tex is shot in the back and drowns. As Jenny pulls away, a chain of explosions obliterate the Poseidon in a plutonium cloud, along with Svevo and his crew. On the journey home, Mike tells Celeste that without the fortune, the bank is going to confiscate Jenny as soon as he reaches port. Celeste unbuttons her shirt to reveal a diamond tucked away, and Mike kisses her.