Kyros, a merchant to whom Hercules owes money, threatens to kill Medusa unless his debtor travels to the Underworld to retrieve a box. After consulting a servant of Persephone, the bride of Hades, King of the Underworld, Jason, Hercules and Pythagoras discover a means by which they may descend into Hades and return to the world of the living.
A potion that slow their heart rates enables Jason and Hercules are able to travel to Hades after making a sacrifice, while their friend stays behind to revive them with his own blood when they are ready to return.
However when the apparently lifeless forms of Jason and Hercules are caught in a house fire, they are removed for burial while Pythagoras is out cold after falling from the window. Meanwhile, in Hades, they learn that the item they are to retrieve is none other than Pandora's box, which contains all the evils of the world.
Pythagoras finds the graves of Hercules and Jason and revives them, rescuing them from the Underworld in the nick of time. A duplicate of the box is made to give to Kyros and Medusa is rescued but, when she finds the real box, she opens it and becomes the victim of a terrible curse. Consulting the Oracle, Jason leanrs that the curse cannot be lifted, and that his destiny is one day to kill Medusa in order to save Atlantis.
