As the stricken luxury liner S.S. Poseidon continues to float amidst the wreckage, six survivors have already been rescued by the French Coast Guard. In the midst of the chaos, tugboat captain Mike Turner, accompanied by his loyal second mate Wilbur and passenger Celeste Whitman, embarks on a daring adventure to claim salvage rights. Their determination is fueled by the loss of their cargo on the ill-fated Jenny, which also succumbed to the tsunami that overturned the Poseidon.
Soon after, Dr. Stefan Svevo and his self-proclaimed Greek Orthodox medical team arrive, asserting they intercepted the ship's distress signal. As they bravely board through a massive opening caused by the French rescue team, they discover the infernos raging in the engine room, the twisted red valve wheel once held by Reverend Scott, and the heart-wrenching remnants of Linda Rogo, whose disastrous fall had been portrayed in The Poseidon Adventure.
The medics' arrival brings a new sense of urgency, suddenly shattered by a violent explosion from the ship's depths. [Wilbur] warns [Turner] that the Poseidon has become a floating time bomb; however, [Turner] remains resolute in his mission. Navigating the perilous vessel, they encounter more survivors, including [Gina Rowe] (the ship's nurse), the elegantly dressed [Suzanne Constantine] (/actor/shirley-jones), and war veteran Frank Mazzetti, whose desperate quest is to locate his missing daughter [Theresa] (another survivor driven by paternal love).
Eventually, [Theresa] is found alongside elevator operator [Larry Simpson] and the mysterious "Tex," who is tightly gripping a valuable bottle of wine. They also come across [Harold Meredith] and his wife [Hannah], who have patiently awaited rescue. Meanwhile, water continues to flood the decks, interrupted by additional explosions that throw the ship into further disarray.
During their search in the purser's office, [Svevo] splits from his team, leaving the remaining survivors unattended. Soon, another explosion causes the safe to burst open, revealing a wealth of gold coins, diamonds, and cash. [Turner] and [Wilbur] quickly seize the opportunity to snatch the treasure.
Unbeknownst to [Turner] and the others, [Suzanne] has been clandestinely collaborating with [Svevo], stealing an essential document related to an unlisted cargo before returning to the group. However, her betrayal ultimately seals her fate. [Svevo] orders one of his henchmen, [Doyle], to eliminate her. In a desperate twist, as [Suzanne] lays dying, she manages to strike [Doyle] with an axe, avenging herself in her final moments.
As [Turner] and his group ascend through the Poseidon's decks, they encounter [Suzanne]'s lifeless body, realizing that they are not the only ones fighting to survive—a killer is lurking among them, poised to strike again. Amidst the turmoil, [Hannah] strains her shoulder while attempting to support her husband, as [Svevo]'s nefarious plans are brought to light: he has been concealing a covert stockpile of plutonium on board the Poseidon. His true intentions now revealed, [Svevo] asserts that [Turner] and his group are no longer allowed to leave, but fate deals another hand as another explosion sends shockwaves through the ship, granting [Turner] and his companions a chance to escape.
In a fierce struggle, [Turner] (along with [Mazzetti] and [Simpson]) confronts [Svevo]'s henchmen in a frantic shootout. The cost is devastating: both [Mazzetti] and another companion meet their grim fate, while [Hannah], left behind in the tumult, succumbs to her injuries as the rising waters claim her life. In a desperate effort to save his wife, [Turner] loses the precious treasure he had salvaged, serving as a painful reminder of the catastrophic ramifications of [Svevo]'s treachery.
Meanwhile, [Svevo] and his remaining loyalists persist in trying to recover their plutonium cargo from the sinking vessel. As the stern of the Poseidon slips beneath the waves, groaning in agony, [Turner] and the other survivors make their way toward an underwater exit door, bracing themselves for a last-ditch effort for survival.
In a moment of bravery, the gravely injured [Wilbur] sacrifices his life to ensure the escape of [Turner]'s group, disappearing into the abyss as they swim towards the tugboat. During the chaos, [Tex]—a sommelier disguising himself as a wealthy passenger—clings desperately to his wine bottle, meeting a tragic end as he becomes another victim in the unfolding chaos.
In a heart-racing conclusion, the remaining passengers of the Poseidon manage to board the tugboat, just as the ship detonates in a cataclysmic explosion, sinking stern-first into the depths of the ocean. [Svevo] and his loyalists face their doom, consumed by the very disaster they sought to exploit.
As the sun sets, [Turner] reflects on the loss of his tugboat, but hope emerges when [Celeste] produces a salvaged diamond from the wreckage of the Poseidon. In a tender moment, they share a kiss as their vessel sails off into the horizon, leaving behind a somber tally of ten survivors who have escaped the abyss of despair and tragedy.
