Plot
Aboard the ocean liner S.S. Queen, sailing from Puerto Rico to New York, Lieutenant Frank Howard (Jason Robards Sr.) travels in custody, handcuffed and accused of the murder of his wife. He is escorted by Detective R.L. Jackson (Robert Emmett O’Connor), a determined officer of the law. During the voyage, Howard meets Dorothy Whitlock (Virginia Valli), a wealthy young society woman traveling with her aunt, Mrs. Renwick (Clarissa Selwynne). Dorothy is instantly drawn to the quiet, stoic Howard, though the accusation against him troubles her deeply.
As the Queen approaches the treacherous Sargasso Sea, known for its thick seaweed masses and drifting shipwrecks, disaster strikes. The liner collides with a derelict vessel and begins to take on water. Chaos erupts as passengers abandon ship. In the frenzy, Jackson forgets his handcuffed prisoner, still below deck. Realizing his mistake, he races back to rescue Howard.
They return topside only to find the lifeboats gone, too far out to reach. The last lifeboat to have left, however, capsizes and when Howard and Jackson look for any survivors they see a woman floating in the ocean. Howard quickly jumps into the ocean to rescue her. As he brings her back to the ship they discover the woman is Dorothy. The Queen, despite her damage, remains afloat, and the trio begins to drift—eventually carried deep into the heart of the Sargasso Sea. After days adrift, they become stranded amid a surreal floating graveyard of wrecked ships from various centuries—an isolated, decaying “island” of lost vessels and seaweed.
To their astonishment, they discover a colony of more than fifty shipwreck survivors, all men save for two women. The strange society is ruled by Captain Peter Forbes (Noah Beery Sr.), a grizzled ex-whaler who enforces a harsh law: any woman who arrives must marry at once to avoid disputes among the men. Forbes, having long been without female company, claims Dorothy for himself.
Dorothy, however, chooses Frank Howard. Under the colony’s law, Forbes has the right to challenge her choice. A brutal hand-to-hand duel ensues between Howard and Forbes. Howard triumphs, earning the right to marry Dorothy. But the tension does not abate—Forbes and his followers vow revenge, and Detective Jackson, now more ally than jailer, joins Howard and Dorothy in preparing for a possible attack.
Their sanctuary becomes the partially sunken S.S. Queen, which they fortify for a final standoff. However, Mr. Burke (Robert Homans), a friendly Irish mechanic who powers the colony’s electric lights using a stranded submarine, reveals a better way out: the submarine is fully functional. Burke secretly leads them to it, and Howard prepares for escape.
As they board the vessel, Forbes and his men arrive with rifles, firing at them and even managing to lasso the sub. A tense moment follows, but the submarine breaks free and dives beneath the seaweed, escaping the floating nightmare of the Sargasso Sea.
Now safe and at last beyond the reach of the lost ships, Jackson agrees to help Howard clear his name back in Puerto Rico. Dorothy, relieved and in love, tells Frank that their sham marriage aboard the derelict wasn’t a formality—it’s going to be a real one.
