When villagers begin disappearing it is discovered that they had fallen through a crack in the earth which goes straight to Hades. Hercules once again comes to the rescue and faces one of his most difficult challenges, and must prove himself as a man as well as a god.
In the centre of a village, the ground begins to open up and a strange green light emanates from within. Two drunken men see the light and walk over to take a closer look. As they approach, a gaseous vapour begins pouring out of the fissure and in a flash of light the two men are charred with only their bones remaining. The next day in a village square, a man challenges the villagers to fight Eryx the boxer. One man agrees but he is tricked when he is introduced to the real boxer, a towering brute of a man. The challenger dies fighting Eryx, so an old man from the village tells a youth to find Hercules. After a short while, the boy returns with Hercules, who challenges Eryx. They begin to fight and it appears that Eryx is going to beat Hercules, but then Hercules finally ends the fight by killing Eryx. With the man dead, the man who first made the challenge gives Hercules a peacock feather, Hera's symbol. He goes to Hera's temple and asks if they can call a truce; Hera defies Hercules, so he destroys her temple. Zeus appears and tells Hercules that he will only make things worse between him and Hera. When Hercules arrives home, Deianeira tends to his wounds.
The following day, Hercules is working in the smithy with Nessus, the centaur. He watches the children playing outside when a woman, Iole, comes looking for him. She says she's from the village of Gryphon and that they need Hercules' help. Hercules agrees to help, but Deianeira tells the girl to rest first. During the night, Iole tells Deianeira that she thought she saw someone outside her window. Deianeria tells her no one is there. Deianeira gets a lantern and goes outside; she finds Nessus in the smithy and he tells her that she cannot trust Hercules with Iole as she is a young woman, Hercules will not be able to resist. She defends him saying that Hercules would be faithful to her. In the morning Hercules, Iole and Nessus leave for Gryphon. Before leaving Iole gives Deianeira a necklace to thank her for looking after her the night before. Deianeira goes to the market where a woman tells her that the necklace is a sign that she has lost her husband and tells Deianeira about the necklace is given to women whose men are to be killed by Nurian maidens. Deianeira goes after Hercules to warn him and finds the three at the river bank. She tells Iole to leave, but Hercules says he already knew she was a Nurian maiden, but that he loves Deianeira, and would never be unfaithful to her. After reassuring Deianeira, Hercules and Iole continue to Gryphon, but Nessus begins to stir doubts in Deianeira's mind, and after she tries to get away he attacks her. She calls for Hercules, who shoots an arrow which strikes Nessus in the back. As he lay dying, Nessus tells Deianeira that his blood is powerful and will prevent Hercules from being unfaithful. She gives the cape to Hercules and tells him to wear it if he gets cold.
Hercules and Iole continue onto Gryphon, when they arrive the people take him to the area where the hole has opened up in the ground. He walks through the village seeing fire and destructions and dead bodies strewn on the floor. He approaches the fissure and looks into it and see spirits coming out from deep within the Earth. Zeus appears and tells him that it is the Underworld. Hercules asks if he is mortal or not, Zeus tells him he is mortal, but tries to prevent Hercules from going down the hole. Hercules prepares to go into the hole and puts on the cape that Deianeria gave him. It begins to choke him and tries to kill him, he struggles free and throws the cape, which disappears leaving only a peacock feather. Hercules runs and jumps into the hole. As Hercules travels to the Underworld, a man arrives at Hercules' house and tells Deianeira that Hercules is dead. He explains about the cape trying to kill Hercules and that he jumped into the hole. Hercules arrives in the Underworld, where he meets Charon, who transports him across the River Styx. On the other side of the river Hercules finds Cerberus' collar, he enters a doorway and vanishes.
Meanwhile, Deianeira, distraught by the thought that she caused her own husband's death, goes to a cliff top; while standing there she sees a vision of Hercules and reaches out to him; as she reaches out she falls from the cliff to the rocks below. After being attacked by different monsters, Hercules meets Eryx the boxer and some other people he sent to Hades. He fights Eryx again and then sees Nessus, who taunts him by showing him, via a portal, that Deianeira is dead. Hercules ask for Nessus to show him again, when Nessus shows Deianeira again Hercules jumps through the portal into the Elysian fields. He finds Deianeira but she has no memory of him, Hades appears and tells Hercules that he erased her memory about Hercules because of the thought of killing her husband. He begs Deianeria to remember him and their children and then kisses her. With the kiss her memories return and Hercules makes a deal with Hades that if he can capture Cerberus, then Deianeira can go back to Earth with him. Hercules goes after Cerberus, he finds Hades' men trying and failing to capture him. Hercules fights and defeats Cerberus and chains him up again. Once Cerberus is chained the hole in the ground closes up and Deianeira appears. Back on Earth the villagers thank Hercules for helping them and he and Deianeira go home.
Nessus, Hades, and the Cerberus all play into the labors of Hercules, although not in the fashion portrayed here. The capture of Cerberus was supposedly the last labor of Hercules where he brings Cerberus to the surface world at the demand of King Eurystheus, not a request of Hades. He returned Cerberus to Hades only after the twelfth labor was completed.
The scene where Deianeira was abducted by the Centaur and where Hercules shot him with an arrow were supposedly the last moments of Hercules as a mortal. When he was caught by the fire, Hercules chose to be cremated alive to hasten his death but was transformed by Zeus into a full god. Also, Deianeira is Hercules' second wife and her death was by suicide; Megara was his first wife.