Hercules is on the road to the court of King Pelias of Iolcus to tutor Pelias' son Prince Iphitus in the use of arms. Pelias' beautiful daughter Princess Iole updates Hercules on the history of her father's rise to power and the theft of the kingdom's greatest treasure, the Golden Fleece. Some suspect—and it eventually proves true—that King Pelias has acquired the throne through fratricide. Hercules and Iole are attracted to each other and a romance eventually develops.

King Pelias is warned by a seeress about a stranger wearing one sandal who will challenge his power. When his nephew Jason, the rightful King of Iolcus, arrives in town wearing one sandal, Pelias takes fright and packs him off to retrieve the Golden Fleece from the distant land of Colchis. Jason and Hercules sail aboard the Argo with their friends Ulysses and his father, Laertes, Argos, the twins Castor and Pollux, the lyre-strumming Orpheus, the physician Aesculapius and others.

After weathering a tempest at sea, the Argonauts dally in a lush garden-like country with Antea, the Queen of the Amazons and her ladies. Jason falls in love with Antea, but, when the Amazons plot the deaths of the heroes, Hercules forces Jason to board the Argo and secretly set sail in the night. On the shores of Colchis, the heroes battle hairy ape-men while Jason slays a dragon and retrieves the Golden Fleece. The Argonauts embark for home with their prize.

In Iolcus, the populace greet the returning heroes but Pelias and his henchman Eurysteus steal the Golden Fleece, deny Jason's claim, and plot his destruction. A tense battle between Pelias' forces and the heroes follows. Hercules halts Pelias' cavalry dead in its tracks by toppling the portico of the palace upon them. The defeated Pelias drinks poison. Jason ascends the throne while Hercules and Iole set sail for new adventures.

Subplots involve the death of Pelias' headstrong son Prince Iphitus, and exploits for Hercules resembling the Labors of the Nemean Lion and the Cretan Bull.