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. When he is a grown man, Ulysses uses his life savings to purchase the
Mare Nostrum, a fast, modern freighter, and prospers. However, he finally gives in to his wife, Doña Cinta, for the sake of their son Esteban, and agrees to sell his ship. With the outbreak of
World War I, however, the enormous profits to be made from the sudden demand for shipping ends this plan. On a stop in Italy, Ulysses visits the ruins of
Pompeii, and meets Freya Talberg and the learned Doctor Fedelmann. He soon falls in love with Freya (who looks exactly like his uncle's painting of Amphitrite). Though she later informs him that she is an
Austrian spy (as is Fedelmann), Spain is neutral and his ardor is undiminished. He agrees to transport Count Kaledine to a secret rendezvous in the Mediterranean. The
U-boat U-35 surfaces, takes on fuel from Ulysses' ship, and departs with Kaledine. Meanwhile, young Esteban leaves home without permission to find his father. After a week waiting for Ulysses at his lodgings, Esteban goes back to Barcelona aboard the
Californian, a British passenger ship. However, the boy is killed when the
Californian is sunk by the
U-35. Ulysses learns of his son's fate from a survivor, and realizes to his grief his role in the tragedy. He vows to avenge his boy. Upon hearing of the death, Freya sends Ulysses a letter denouncing the barbarity of the act; it is intercepted by Doctor Fedelmann. That, along with Freya's admission she has fallen in love with Ulysses, convinces Fedelmann that her subordinate can no longer be trusted. She sends Freya to
Marseille, intending to betray her to the French. Freya suspects as much, and begs Ulysses to take her to safety aboard his ship. Ulysses is torn, but a vision of his son shaking his head makes him refuse. Freya is later captured, convicted, and shot by a
firing squad at dawn. As he is leaving Freya's apartment, Ulysses encounters Count Kaledine. After a brief struggle, he chases Kaledine through the streets, gathering a mob. Kaledine is caught and taken into custody. Ulysses then employs the
Mare Nostrum in the service of the Allies, arming her with a
deck gun, replacing his crew with French military sailors, and transporting munitions to
Salonica. Only longtime family friend and sea cook Caragol refuses to leave him. On the voyage, they are intercepted by the
U-35. With the
Mare Nostrum torpedoed and doomed, Ulysses mans the abandoned
deck gun and sinks the
U-35. As Ulysses descends into the ocean depths, Amphitrite rises to embrace and kiss him. ==Cast==