Twenty years after Pelios murdered King Aristo for the throne of Thessaly, Jason (Todd Armstrong) returns home to claim the kingdom that is, by all rights, his. Yet, in order to do so, he must first find the Golden Fleece. So he builds a ship, The Argo, and selects a crew before sailing to far-off Colchis in search of the prize. But, in a move intended to foil Jason's plans and seize the Fleece for his father, Pelios's son, Acostus (Gary Raymond), is sent along on the voyage, too. With Hera, Queen of the Gods, helping him to overcome the many obstacles placed in his path, Jason rescues Medea from the sea. Without revealing her identity, she directs him to her father, King Aeetes (Jack Gwillim), the ruler of Colchis. But, forewarned by Acostus of their plans to steal the Golden Fleece, after receiving them in friendship, Aeetes then condemns Jason and his men to death. Torn between love for Jason and loyalty to her father, Medea frees him and leads him to the Fleece, which is in a glade guarded by a seven-headed Hydra. Destroying the creature, Jason takes the Fleece and, fleeing with Medea and his men, eludes Aeetes and his soldiers to begin the long voyage home aboard the Argo.