The S.S. Poseidon, an ocean liner slated for retirement and scrapping, makes her way across the North Atlantic Ocean and the Mediterranean Sea from New York City to Athens. Despite protests from Captain Harrison (Leslie Nielsen), who fears for the ship's safety, the representative of her new owners, Mr. Linarcos, insists that he should go at full speedâ26 or so knotsâtowards Athens to save money, preventing him from taking on additional ballast.
The Reverend Frank Scott (Gene Hackman), a heretical minister believing God helps those who help themselves, delivers a sermon. Detective Lt. Mike Rogo (Ernest Borgnine) and wife Linda (Stella Stevens), a sarcastic former prostitute, deal with her seasickness. Susan (Pamela Sue Martin) and younger brother Robin (Eric Shea) are traveling to meet their parents. Robin is interested in how the ship works and frequently visits the engine room. Retired Jewish hardware store owner Manny Rosen (Jack Albertson) and wife Belle (Shelley Winters) are going to Israel to meet their two-year-old grandson for the first time. Haberdasher James Martin (Red Buttons) is a love-shy, health-conscious bachelor. The ship's singer, Nonnie Parry (Carol Lynley), rehearses for the New Year's Day celebration with her band.
That evening, passengers gather in the dining room to celebrate. Harrison is called to the bridge in response to a report of an undersea earthquake. He receives word from the lookout that a huge wave is approaching from the direction of Crete, at 60Â mph. He issues a mayday distress signal and commands a "hard left" turn, but it is too late. The wave hits the ship and she capsizes.
In the dining room, survivors take stock of their predicament. Acres (Roddy McDowell), an injured waiter, is trapped at the galley door now high above. Scott surmises that the escape route will be found "upwards", at the outer hull, now above water. Robin tells him the hull near the propeller shaft is only one inch (2.54Â cm) thick. The Rosens, the Rogos, Susan, Robin, Acres, Nonnie, and Martin agree to go with Scott, using a Christmas tree as a ladder. Scott unsuccessfully tries convincing more passengers to join them. After the group climbs to the galley, there is a series of explosions. As seawater floods the room the survivors rush to the Christmas tree, but the weight of everyone climbing causes the tree to fall. The water fills up the dining room and the Poseidon begins to sink.
Acres and Scott find the galley, and the survivors make their way to a staircase. Scott climbs its underside, then he and Rogo use a fire hose to pull the others up, with the water quickly rising behind them. Scott leads the group to an access tunnel. While climbing a ladder inside a funnel, the ship rocks from another series of explosions. Acres falls and is lost.
Climbing out of the shaft, the group meets a large band of survivors led by the ship's medic, heading towards the bow. Scott is certain they are heading for their doom, but Rogo wants to follow them and gives Scott 15 minutes to find the engine room. Although he takes longer than allowed, Scott is successful.
The group discovers the engine room is on the other side of a flooded corridor; someone must swim through with a line to help the others. Belle, who reveals herself to be a former competitive swimmer, tries to volunteer, but Scott refuses her and dives in. Halfway through, a panel collapses on him. The survivors notice something is wrong and without any choice, Belle dives in. She frees Scott and they make it to the other side. While Scott secures the lifeline, Belle suffers a heart attack. Before dying she tells Scott to give her "Chai" pendant, representing the Hebrew sign for life, to Rosen, who in turn will give it to their grandson.
Rogo swims over to make sure Belle and Scott are all right, then leads the rest over. When Rosen finds Belle's dead body he is unwilling to go on, but Scott gives him her pendant, reminding him that he has a reason to live.
Scott leads the survivors to the propeller shaft room's watertight door, but there is another series of explosions, and one of them causes Linda to fall to her death. An infuriated and heartbroken Rogo blames her death on Scott. One of the explosions rupture a pipe that releases steam, blocking their escape. Scott rants at God for the survivors' deaths. He leaps and grabs onto the burning-hot valve wheel to shut off the steam, then tells Rogo to lead the group before letting go of it, sacrificing himself to the fiery waters.
Rogo leads the remaining survivorsâRosen, Martin, Nonnie, Susan, and Robinâthrough the watertight doors and into the propeller shaft tunnel. They hear a noise from outside and bang on the hull to attract the rescuers' attention. The rescuers cut a hole through the hull. It is revealed that none of the passengers that headed toward the bow were rescued. The rescue crew assist the group out of the ship, and the six survivors are flown to safety by helicopter.
Harrison was called to the bridge in response to a report of what?
undersea earthquake