While Hercules is away, his village is plundered and his wife is killed by the army of Ecalia as the first part of a scheme by the treacherous Licos to gain the throne of Ecalia for himself. Licos plans that Hercules will come to Ecalia for vengeance and murders King Eurysteus himself to save Ecalia from his revenge. Hercules learns of the murder of his wife and seeks vengeance, but is thwarted when the King's daughter and heir to the throne, Queen Deianira, offers herself rather than her realm of Ecalia as the guilty party. In accordance with the law, Deianira must face the wrath of Hercules; she is bound to a wall as Hercules throws axes at her that miss Deianira proving her innocence in the eyes of the gods.
Hercules admires Deianira and her bravery. When escorting Deianira back to her capital they come across a band of peasants who have been attacked by a monster. As Hercules seeks the monster their cattle is stampeded and Hercules kills a wild bull with his dagger. Arriving in the city Hercules discovers Deinaira is betrothed to Achilles and leaves.
The resourceful Licos hatches another scheme where he murders Achilles with the dagger Hercules left behind in the bull; not only casting guilt on Hercules but making Deianira available for marriage to him. The actual murderer, Philoctetes, is sent by Licos to the Underworld with the plan that Hercules can only prove his innocence by going to the Underworld where no mortal has ever returned from due to the monstrous Hydra.
Philoctetes is killed by the Hydra. Hercules kills the Hydra, but his battle weakens him into unconsciousness, and he's rescued by the Amazons, ruled by Queen Hippolyta. Hippolyta turns her male partners into living trees after making love to them, but Hercules is only interested in Deinaira. Angered that a man is not interested in her but determined to make love to Hercules and turn him into a tree, Hippolyta's advisor suggests the only way she can gain the attention of Hercules is to change her face and body through magic into resembling Deianira. Hercules manages to escape with the help of the Amazon Némée, while Hippolyta is done in by one of the trees. Hercules eventually returns to free Queen Deainira and Ecalia.
Attempting to escape with Ecalia, Licos is strangled by the monster, Alcione, who is in turn killed by Hercules as he rescues Deianira
- The "Garden of Damned Tree-Men" originally appeared in Book 3 of Virgil's epic poem The Aeneid. The imagery was later used by Dante Alighieri in his narrative poem The Divine Comedy to represent the Wood of Suicides.
- Though she gained fame a "blonde bombshell", Jayne Mansfield does not appear with blonde hair in this film. Her various characters have hair that is black, red/orange, and purple.
- At the time the film was shot, Mansfield was four months pregnant.
- The scene where Hargitay wrestles a bull was prepared by treating the animal with tranquilizers first.