American David Ross (William Holden), a former tugboat captain now in the Canadian army, is hastily commissioned in the Royal Navy and assigned to rotating command of W88, a double-crewed rescue tug then in dry dock due to battle damage. His predecessor was a suicide. The slow, poorly armed tugboats bring in "lame ducks," freighters crippled near England by German attacks. The main danger is from U-boats and aircraft.
The legendary empire of the lost continent of Mu reappears to threaten the world with domination. While countries unite to resist, an isolated World War II Captain has created the greatest warship ever seen, and possibly the surface world's only defense.
The film begins with Commander Bolton and a few surviving crew members of his 50-man submarine Gauntlet swimming ashore after unsuccessfully attacking German battleship Lindendorf. After a review, Captain Bolton is cleared of any wrongdoing and placed in charge of a small group of experimental X class submarines. His mission is to quickly train crews to man the submarines and sink the Lindendorf while it is hidden away in a Norwegian fiord.
Yves, a fisherman, comes home after a tiring day of fishing and soon falls asleep. In his dream, he is visited by the Fairy of the Ocean, who leads him to a submarine. Yves is made Lieutenant-in-Command and sets off on a submarine voyage.
Three men (Dr. Ken Tashiro, Dr. Jules Masson, and Perry Lawton) are trapped in a bathysphere due to seismic activity. They are rescued by the crew of the supersubmarine Alpha, captained by Craig Mackenzie (Cotten), who they learn is over 200 years old (and that the Alpha was launched in the early 19th century). Mackenzie takes them to Latitude Zero to deal with the serious injuries of Dr Masson. While returning to Latitude Zero, they are attacked by a rival supersubmarine, the Black Shark, captained by Kuroi, who works for a rival of Mackenzie, Malic, who is also over 200 years old and has a base at Blood Rock.
The plot revolved around the recent incident involving the German commander Baron von Luxburg who sank an Argentine ship intending to frame the Entente for the act. Reports from survivors helped everyone to realize what had truly happened.
Kaiser Wilhelm II of Hohenzollern (Rupert Julian) is a vain and arrogant tyrant eager for conquest. When Belgium is invaded by the German army during World War I, Marcas, the blacksmith (Elmo Lincoln), although wounded, is able to save his daughter from the clutches of a German soldier. Soon after this, the RMS Lusitania is sunk by Captain von Neigle (Nigel De Brulier), who ultimately is driven mad with remorse. After the United States declares war, the Allied generals turn the Kaiser over to Albert I of Belgium. Incarcerated, the Kaiser faces his jailer, Marcas the blacksmith.
As a young boy growing up in a Spanish family with a long and very distinguished maritime tradition, Ulysses Ferragut is regaled with tales of the sea by his retired uncle, the "Triton" (Apollon), and is particularly fascinated by his claim to have once seen the sea goddess Amphitrite. Though his lawyer father, Don Esteban, wants him to follow in his footsteps, Ulysses becomes a sailor.
Two sailor buddies have their friendship torn apart after the woman they both are in love with chooses one over the other. Their relationship gets re-evaluated when one of them becomes trapped in a submarine and the other gets sent on the rescue mission.
Navy Lieutenant Greg Winters (Alan Hale) is found guilty by a court-martial for pausing briefly to prepare to rescue survivors of the Alatania, a torpedoed ship, rather than attacking immediately the submarine responsible. As a result, he is sidelined for the rest of World War I.
When the commander of the United States Navy submarine AL-14 is wounded on its last cruise, Lieutenant Thomas Knowlton (Robert Montgomery), the second in command, hopes to be promoted and take his place. However, Lieutenant Commander T. J. Toler (Walter Huston) shows up and takes over.
L'équipage d'un sous-marin allemand sous le commandement du capitaine-lieutenant Liers est de nouveau en mer pendant la Première Guerre mondiale. Les hommes ont eu du mal à dire au revoir à leurs compagnes, mais le devoir les appelle. La mère de Liers a déjà perdu deux fils dans la guerre et redoute sa disparition.