During his military service, the young singer Gianni Traimonti falls in love with Carla, the marshal's daughter.

King Androcles of Thebes commands a ship in search of a threat to Greece, predicted by an atmospheric oracle. Hercules is part of the crew, but the strongman falls overboard in a thick fog. Washed ashore, he saves a young Ismene from Proteus, an evil, shape-shifting creature. Ismene takes Hercules to Atlantis, where he discovers that her mother, Queen Antinea, has captured Androcles. Hercules schemes to rescue him and thwart Antinea's dream of world conquest.