The king of Acalia has died. His daughter Princess Dianara is the rightful heir to the throne, but her uncle, Lycos, has seized power and now reigns. He has secretly put Dianara under a spell and keeps her locked out of sight. The only one who can save her is the immortal son of Zeus, Hercules, who loves her. Lycos has sent a band of assassins to kill Hercules upon his return from battle, but they proved unable to overpower the mighty hero. One of the henchmen relays this news to Lycos. When the henchman demands to be paid anyway for his efforts, Lycos has him killed.
Hercules arrives in Acalia, where he is welcomed by a cheering crowd. Kyros tells a surprised Hercules that Dianara has not yet been crowned. Hercules goes to Lycos for an explanation. Lycos explains that Dianara has taken ill and that he has taken control for her own good, so that her people would not discover that she is unfit to rule. Hercules asks to see her. When they meet in the palace garden, Dianara doesn’t recognize him. Kyros nervously tells Hercules that fear has taken over the land and that he must save Dianara; he arranges to meet Hercules in secret so that he can explain more. But when Hercules arrives at Kyros’ house, the man has been murdered.
Not realizing that Lycos is behind all of the evil that befallen Acalia, Hercules consults the oracle Medea, offering to give up his immortality in exchange for her wisdom. She tells him that he must find a living stone in Hades which will cure Dianara. But in order to ensure a safe return from Hades, he must first obtain a golden apple from the Hesperides, women imprisoned by Pluto, god of the underworld, in a land of darkness.
Joined by Theseus and Telemachus, Hercules sets sail toward a land of endless midnight, where they are overcome by sleep. They awaken in the garden of the Hesperides where Zarathusa, their queen, shows Hercules the sacred tree that holds the golden apple. Hercules climbs the gigantic tree in order to obtain the treasure, a task that tests all of his strength. Meanwhile, Pluto has forced the Hesperides to deliver Theseus and Telemachus to the monstrous Procrustes, who kills his victims by either stretching them or cutting their legs to fit the beds they lie upon. By winning the golden apple, Hercules frees the Hesperides from Pluto's captivity. They tell him that his friends are in danger. Hercules discovers them just in time to save them. By destroying the monster, Hercules opens a portal to the underworld.
Hercules and Theseus enter Pluto’s realm, where they discover the magic stone surrounded by a lake of boiling lava. Hercules devises a way to obtain the stone, but as the heroes make their way toward their prize, Theseus falls into the lava and disappears from view.
Theseus awakens in the underworld, tended by a beautiful woman who knows how he can escape the land of the dead. They fall instantly in love, and he determines to take her with him. Theseus is reunited with his friends and they set sail for home with both the golden apple of the Hesperides and the living stone from Hades. Theseus has hidden the woman below decks. When a terrible storm threatens to sink the ship, she tells Theseus that Pluto is angry, and that the golden apple must be thrown overboard. Theseus does so, and the storm instantly subsides.
Returning to Acalia, the adventurers see that chaos has overtaken the land. Theseus’ mysterious beauty reveals that she is Persephone, daughter of Pluto, and that the destruction around them is the result of her father’s anger. Theseus vows that they will never be separated, even if that means that he must defy Hercules. Meanwhile, Hercules brings the living stone to Dianara’s bedside and stands guard over her as she begins to recover.
Lycos is now revealed to be in league with the god of evil. He strikes a deal to gain immortality by killing Dianara and drinking her blood during a lunar eclipse. First, however, Lycos must separate Dianara from Hercules. He tells Hercules that the people blame the hero for incurring Pluto’s wrath upon them; the king urges him to consult once again with the oracle to resolve the turmoil around them. Lycos promises to have Dianara guarded, but after Hercules leaves, he kills Dianara’s handmaiden Helena, then drags the princess away for the unholy sacrifice.
Medea tells Hercules that, in order to save Acalia, Theseus must renounce Persephone and send her back to her father in the underworld. When Hercules asks Theseus to sacrifice his own happiness for the sake of Acalia, the two come to blows. Hercules easily overpowers his friend, who falls unconscious. Persephone now intervenes. Knowing that she and Theseus have no hope of future happiness because of her father’s anger, she promises Hercules that she will leave of her own accord, returning the living stone to her father’s realm, if he will spare Theseus’ life. She also warns the unsuspecting hero that Lycos plans to kill Dianara. As Hercules rushes back to the palace, Dianara kisses Theseus one last time, erasing his memories of their time together, and disappears.
As the lunar eclipse begins, Hercules searches the palace for Dianara. In the catacombs, he watches as an army of the dead emerges from the tombs. He escapes them, and comes upon Lycos, who is about to sacrifice Dianara. Although Lycos displays unnatural strength, Hercules overcomes his mortal enemy and vanquishes the advancing legion of ghouls. Hercules prays to Pluto to leave Acalia in peace. He and Dianara are happily reunited at last.