Iolaus arrives in the city of Argos, where the Argonauts are having a party. He is greeted by Otus, and Castor. The party is being held because it is the tenth anniversary of the Argonauts bringing back the Golden Fleece. Iolaus is introduced to Phoebe, the daughter of Lycenus, the lookout. He says he is sorry about the death of her father, but she says her father's death was no accident. Meanwhile, Hercules meets a gang of robbers mugging a man. Hercules fights them off and discovers the man is Jason, who hallucinates that Hercules is a horned figure, come to take him "to the fire below".
Hercules brings Jason to the party and they discover that Jason is no longer able to run the kingdom by himself and has appointed a Regent in his place to run the kingdom of Argos. The reason why Jason is unable to rule is because of his drinking problems. He took to drinking because he left Medea for Glauce, bitter with jealousy, Medea killed Glauce and Jason's children.
During the party, Jason sees the horned figure again, but then it disappears. As he goes to get a drink from the barrel, a man leaps from the barrel and more break in through the windows. As a fight ensues, the horned figure comes and steals the Golden Fleece. Hercules identifies them as Hera's warriors.
Iolaus tells the other that he tracked them to the harbor, where they were sailing east. Hercules says they will get the Fleece back and that he wants Jason to navigate the Argo. As they sail off in the Argo, the regent, Marcus meets with a priestess of Hera, who gives him a bag in case Hercules and the Argonauts succeed in regaining the Fleece.
When the ship reaches land, they catch up with Hera's men and prepare to attack. Jason leads the attack on the camp, as they fight, their leader the man in the horned helmet, flees. Jason follows and the two men fight. As the leader is about to kill Jason, Hercules appears and stops him throwing him across the forest and he flees up a tree.
After the fight, one of the Argonauts is found with a stab wound in the back. Artemus says he will kill Jason, before he gets anyone else killed. Hera's men take the Fleece into the side of a volcano, Jason says he will go in after the Fleece by himself, but he insists they all came together, so they will finish it together.
As they enter the cavern, they are confronted by the horned man again, Hercules says, "Take off your mask, Castor". The man removes the mask and reveals himself to be Castor. Castor, consumed with jealousy, after Jason had retrieved the Golden Fleece ten years earlier, he became the demon plaguing Jason's nightmares.
A fight breaks out and the two sides side each other. Meanwhile, the volcano is close to eruption as Jason retrieves the Fleece, but then stops to save Artemus' life and Castor leaves with the Fleece for the second time. With the volcano due to erupt, Hercules suggest they leave and go back to Argos. When they arrive, Marcus tells Jason that he has failed in retrieving the Fleece, and the council wants to depose him and replace him with Castor.
Jason agrees to let Castor have the throne, if he fights him; the winner will be the new ruler of the kingdom. Castor agrees, and Marcus produces some teeth from the bag he was given earlier and tosses them into the fire. From the fire appear skeleton warriors, who begin to fight the Argonauts. Castor goes after Jason and they begin fighting and he kills Castor.
Later, Artemus and Phoebe, approach Hercules and asks that she be made an official Argonaut, Hercules says he would be proud for her to be an Argonauts, Phoebe says that she hopes she can honor the memory of her father, to which Hercules replies that she already has. Hercules finds Jason outside and they both talk about their lost families and they go inside to join the others.