Upon his return to Italy from his many adventures, the great warrior Hercules learns that his lover, Princess Deianira, has insanity. According to the oracle Medea (Gaia Germani), Daianara's only hope is the Stone of Forgetfulness which lies deep in the realm of Hades. Hercules, with two companions, Theseus and Telemachus, embarks on a dangerous quest for the stone, while he is unaware that Dianara's guardian, King Lico, is the one responsible for her condition and plots to have the girl for himself as his bride upon her revival. Lico is in fact in league with the dark forces of the underworld, and it is up to Hercules to stop him.