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Just before New Year's Eve, the ocean liner S.S. Poseidon hits choppy weather at sea. On the bridge, Capt. Harrison argues with Linarcos, the representative of the consortium that now owns the ship, who is insisting that they carry on without ballast in order to make up lost time. When young Robin Shelby bursts into the cabin, he impresses Harrison with his fascination with and knowledge of the ship, but is ushered out to await a safer moment to take his tour of the bridge. In addition to Robin and his eighteen-year-old sister Susan, the ship's passengers include police detective Mike Rogo and his wife Linda, meek haberdasher James Martin, older Jewish couple Manny and Belle Rosen and Reverend Scott, an unorthodox, rebellious clergyman who has been banished to Africa by his church. While the passengers endure the turbulent seas, Harrison orders that they take on ballast but is forced to back down after Linarcos threatens to have him removed from command. In the bar, singer Nonnie Parry rehearses her songs, watched appreciatively by a British waiter named Acres. Later, Scott delivers a guest sermon to the ship's passengers, preaching that God is not concerned with individuals, so everyone must have the tenacity and courage to solve his own problems. That night is New Year's Eve, and the passengers prepare for the ballroom gala. Linda, a former prostitute, worries that others disdain her, and when Mike tries to calm her, they spar briefly, then embrace. At dinner, while Belle worries that Martin is lonely and urges him to find someone to marry, Susan develops a crush on Scott. On the deck, meanwhile, Harrison learns of a subsea earthquake that is causing heavy swells nearby, and he and his crew wait tensely. Just as midnight strikes and the ballroom erupts in celebration, an enormous wall of water slams into the ship. The force of the water turns the Poseidon onto its side and then, as everyone and everything inside the ship slides downward, the ship flips completely over. In the ballroom, passengers are flung against the walls and slide down the floors to the ceiling, which is now below them. Many people are killed or crushed under furniture, while others hang for their lives from the tables, which were bolted into the floor. In the ensuing silence, the survivors rouse and search for their loved ones. Manny and Belle find each other, as do Linda and Mike. Scott tries to help the wounded and Nonnie cradles the dead body of her beloved brother, as the purser announces that they must stay where they are and await help. When Susan, trapped on a table, yells for help, Scott organizes the men to form a net out of a curtain and urges the terrified teenager to jump. After Susan lands safely in the net, Acres, who sits on a high platform with an injured leg, also asks to jump. Martin and Scott determine, however, that because the boat is now upside down, any rescue team will have to come from above them through the hull, thus everyone is safer joining Acres at his higher vantage point. Robin, who has amassed copious data about the ship, informs them that the hull is thinnest near the propeller shaft, prompting Scott to announce that everyone must make their way toward the shaft. Although Mike is reluctant to trust a child and chafes at Scott's officiousness, Linda insists that they go along. Scott leads some men in righting the huge, ornamental Christmas tree, which has a strong inner skeleton that can act as a ladder, and Acres anchors it from above. After the women remove their skirts, Robin, Susan, Linda and Mike climb the tree. Martin tries to convince the others to come along, but only Belle, Manny and Nonnie agree, although Belle is convinced she is too heavy to make it, and Nonnie is in mild shock. Before following Martin up the tree, Scott urges the ship's chaplain to join them, but the older man feels he must remain to comfort the others. Scott addresses them all, stating that climbing represents their only chance to escape drowning as the ship continues to sink, but they side with the purser and refuse to leave. Just as Scott reaches the top of the tree, an explosion rocks the ballroom and water rushes in. Panicked, everyone runs for the tree at once, and their combined weight topples it, trapping them below. With no other choice, Scott closes the door on their cries for help. The party of ten crosses the galley to a fire door, and although Mike is fearful of a flash fire on the other side, Scott pushes through and leads them toward a passageway, which Acres identifies as running the full length of the ship to the engine room. Looking for an entrance to the passageway, the group climbs broken, upside-down stairways. As they struggle to stay steps ahead of the water and debris rushing at them, each group member's strengths and weaknesses are highlighted, with Mike constantly chafing at Scott's domineering attitude, and Nonnie nearly collapsing with despair. When they find the entrance to the passageway sealed, Acres recalls that there is a duct into the central air shaft. They crawl through the shaft to its end, where Scott barely manages to open the door before water floods in. The shaft opens onto a vertical tube housing a steep ladder leading to higher doors. After Scott confirms that the next door can be opened, each climbs up precariously. During another explosion, Acres falls to his death, after which Nonnie freezes in fear and Martin must talk her into continuing to climb. The door leads to a deck of the ship, where they are thrilled to spot other survivors being led forward by the ship's doctor. Scott, however, informs the doctor that he is going the wrong way, toward the section of the ship that is sinking fastest. Despite the doctor's certainty that he is correct, Scott tries urgently to convince the others that the engine room is the only way out. When Scott blames Mike for failing to save Acres, Mike explodes, asking if Scott thinks he is God himself. After Martin breaks up the fight between Scott and Mike, Scott determines to go ahead to the engine room by himself to ensure that it is accessible, with Mike vowing to follow the doctor if Scott does not return in fifteen minutes. While Susan follows Scott, the others wander the deck looking for supplies. Belle tells Manny she hopes the children survive, and hands him her necklace, the Hebrew letter chai, meaning life. As Scott finds a hatch and climbs into it, cautioning Susan to return to the others if he is not back within minutes, Martin urges Nonnie to be strong, and Robin wanders into a bathroom. When Scott fails to reappear, Mike prepares to leave, but as Manny and Martin argue with him, Scott returns with the news that he has discovered the engine room. Robin cannot be found, however, and as another explosion rocks the deck, Scott searches for the boy. Locating him, he grabs Robin and reaches the hatch just moments before the water rises above their necks. The engine room is now one deck below them, and although climbing downward is frightening, they all do so, only to find the corridor now under water. Scott orders them to swim the short corridor, and dives in to lead the way. When the rope tied around his waist goes slack, Belle, a former champion swimmer, hurls herself in to rescue him. Underwater, she frees Scott from the debris pinning him down, and they surface in the engine room. Moments later, however, she suffers a heart attack and dies in his arms. Mike has followed and now Scott demands that he round up the others, revealing nothing about Belle. Nonnie, who cannot swim, holds on to Martin, and all reach the engine room safely. There, Manny weeps while holding his beloved wife, and although he does not want to go on, Scott convinces him that dying will only render Belle's death meaningless. The engine room is racked by fires, but Scott points out that the shattered machinery forms a catwalk on which they can climb to the propeller shaft. They all climb until a blast knocks Linda into the flames. Mike howls in grief, blaming Scott for taking from him the only thing he has ever loved. Another explosion sends plumes of steam up, blocking the propeller shaft door. Exhausted, Scott rails against God, asking how many more sacrifices He will demand. Leaping over the flames to jump onto the red valve that controls the steam pressure, Scott grabs the valve, which is directly under the scalding steam, and tortuously twists the valve closed yelling "You want another life, then take me." Knowing that he has no way back, Scott encourages the others to go on, then lets go of the valve and drops to his death. Susan threatens to jump, but Martin holds her back, then taunts Mike into leading them. The steam now subsided, Mike is able to pull the door open, and the group hammers on the thin hull. An answering bang is heard, and Mike realizes that Scott was right all along. The rescuers use blowtorches to cut a hole in the hull and escort the group, tattered and stunned, to safety.