The SS Poseidon is a transatlantic passenger steamship that is set to retire, embarking on its final journey from New York City to Athens. On the bridge, the company executive Linarcos (Fred Sadoff) instructs Captain Harrison (Leslie Nielsen) to jettison ballast and push the ship to full speed, a decision that deeply concerns the seasoned captain.

As the voyage unfolds, Detective Lieutenant Mike Rogo (Ernest Borgnine) and his ex-prostitute wife, Linda (Stella Stevens), are enduring the challenges of sea sickness that many passengers also face. During a shipboard meal, they receive a prestigious invitation to join the captain’s table, reflecting the importance of the evening. Meanwhile, Reverend Frank Scott (Gene Hackman), a priest wrestling with personal doubts, delivers a thought-provoking sermon that shakes the faith of many aboard during the Sunday service. Among the travelers are young Susan Shelby (Pamela Sue Martin) and her inquisitive younger brother Robin (Eric Shea), who is fascinated by the ship’s mechanics and regularly explores the engine room.

Elsewhere on the ship, retired hardware store owner Manny Rosen (Jack Albertson) and his wife Belle (Shelley Winters) are on their way to Israel to finally introduce their grandson after years apart. Complicating matters is James Martin (Red Buttons), a bachelor who is both love-shy and health-conscious, and Nonnie Parry (Carol Lynley), the ship’s singer, who is busy preparing for the New Year celebrations.

As night falls on New Year’s Eve, revelers come together in the dining room to celebrate. But the atmosphere turns somber when Captain Harrison is urgently called back to the bridge following reports of an underwater earthquake near Crete. In a moment of dread, Harrison is warned of an enormous wave racing toward them. He quickly issues a mayday signal, but the ship is struck by the immense wave before any real action can be taken. Tragically, the ship rolls over, resulting in chaos as many on board are caught off guard, leading to a catastrophic loss of life.

Following the disaster, the remaining passengers and crew find themselves in a precarious position, with the once-stalwart dining room now inverted and serving as their new ceiling. Amid the panic, Robin calls out for his sister, who has become trapped beneath a table. Frank Scott, resourceful and determined, rallies the survivors and devises a plan to cushion her fall using a tablecloth. As hope flutters amidst despair, injured waiter Acres is stranded at an inaccessible galley door now positioned overhead. Inspiring flickers of courage, Scott declares that their best chance for rescue lies “upwards,” toward the outer hull, presenting an opportunity for escape.

As the narrative escalates, the survivors—Manny, Belle, Mike, Linda, Nonnie, Susan, Robin, Acres, and James Martin—form a reluctant alliance under Scott’s leadership to navigate through their peril. They strategize to use a Christmas tree as a makeshift ladder to assist in rescuing Acres. Frank Scott attempts to persuade other passengers to join this desperate endeavor, but many choose to remain, paralyzed by fear in the hope of awaiting rescue.

With rising tension, the group manages to reach the galley, only for more explosions to erupt around them, flooding the ballroom with seawater. The panic-stricken survivors rush toward the Christmas tree as it teeters and ultimately collapses into the rising waters. In a heart-wrenching moment, Scott, the last to leave the disaster-stricken area, closes the kitchen doors behind him, a silent farewell to the passengers he couldn’t save.

Amid the chaos, Scott and Acres navigate to the galley, where they encounter dangers including a fire door sealed shut with fiery heat. With determination and bravery, Scott enters the incinerated kitchen, finding a path forward, as his courage encourages the others to keep pushing through to safety. Together they traverse a series of treacherous terrains, attempting to evade the encroaching seawater and find a way out. The action heightens when Scott and his group seek to cross the flooded engine room corridor, needing someone to swim through rugged waters with a lifeline. Belle, a former swimmer, volunteers but is ultimately saved by Scott just as they’re about to be swept away.

The group faces more harrowing challenges as they race through flooded passages, where tragedy strikes again and again. Belle, upon bravely rescuing Scott, succumbs to a heart attack, uttering her last wish for Scott to pass her cherished “Chai” pendant to her husband. An emotional Scott honors her memory, reminding them all of their shared hopes and lives.

As the tension swells, confrontations flare between Scott and Rogo over the mounting losses endured at sea. While Scott navigates through chaos and injury, the survivors must fend against boiling scalding steam and flames that threaten to sever their chances of life. In an act of sacrifice, Scott clings to a valve wheel, battling the steam while imploring God, only to lose his grip, plunging below as his fate hangs in the balance.

At last, the remaining survivors—Rogo, Manny, Nonnie, Susan, and Robin—finally discover a passage leading them further toward the propeller shaft room, but they find no sign of rescue yet. Their desperation leads them to banging on the metal hull, and in a moment of triumph, they receive an answer. As rescuers cut through, the group is lifted to safety by helicopter, a bittersweet conclusion for the six souls who have triumphed over incredible odds, forever changed by their harrowing experience aboard the SS Poseidon.