During the Hyborean age, "a time of high adventure," a blacksmith, Nial (William Smith
) creates a masterwork sword. The sword is completed, Nial shows it to his son, Conan (Jorge Sanz
), teaching the boy about their god, Crom, who lives in the Earth, and "the Riddle of Steel" which every warrior must solve before they can enter Valhalla. He adds that he should not trust men, woman, or beasts, "But this you can trust," indicating the sword he has crafted.
It is winter in Conan's forest village, home to Conan's people, the Cimmerians. Conan is still young (8-10 years). Mysterious black riders, bearing a strange standard of a serpent with a head on each end surmounted by a black sun and moon suddenly and savagely attack the village. The villagers fight bravely, but they are not soldiers, and do not last long. Nial kills several of the attackers, but eventually felled by the group's attack dogs.
As the brutal attack winds down, Conan's mother Maeve (Nadiuska
) is the only adult still alive. She stands guard over Conan with her own sword, holding the little boy's hand. The leader of the attackers, Thulsa Doom (James Earl Jones
), a mysterious black man with straight hair and piercing blue eyes, approaches the pair with his two hulking lieutenants, Rexor (Ben Davidson
) and Thorgrim (Sven-Ole Thorsen
). Maeve stands her ground bravely but is hypnotized by the piercing eyes of Doom. Doom, having taken Nial's sword from Rexor, suddenly wheels and beheads Conan's mother. A grief stricken and terrified Conan watches as Thulsa Doom keeps the sword his father made. Conan and the village children, the only survivors, are led away in chains as slaves while their village burns.
Conan and some of the children are sold to a tribe called the Vanir and join other children at a gigantic outdoor grain mill, called the Wheel of Pain. They begin pushing the spokes of the wheel, and time passes quickly. The child Conan becomes an adult (Arnold Schwarzenegger
) and the camera pans out to show that he is now the only one left pushing the wheel. Conan has become enormously muscular by the time the mill is shut down and he is led away in chains.
Conan is purchased by a large, red-haired warrior (Luis Barboo
) who employs him as a pit fighter. At first Conan is puzzled by the violence, but quickly learns hand-to-hand combat with a variety of weapons. He becomes a champion, winning much gold for his master. His master then takes him to the Far East where he is trained in all manner of combat skills, weaponry, and philosophy. Finally one evening his master, after having drunk too much, decides to free him.
The freed Conan finds himself being chased by wolves across the country-side. Climbing a cairn of rocks for safety he discovers the tomb of an Atlantean general and liberates his ancient sword and other equipment for protection.
Now equipped with the basics, Conan travels until he comes to the hut of a beautiful woman who appears to be a witch (Cassandra Gava
). She invites him in and she offers to tell him his fortune. Conan says that he's looking for a "standard" (a symbol) of a single serpent with two heads that face each other. Later while they make love she cries out that Conan will find the symbol in the land of Zamora. Moments later, she shape-shifts into a demon and Conan is forced to throw her into the fire. She explodes in flames, turns into a fireball and races around the hut and out through the surrounding woods. Conan is left alone to ponder the incident.
The next morning, he finds a man chained behind the hut. His name is Subotai (Gerry Lopez
) and he is a thief. Conan releases him and they team up. Subotai introduces Conan to thievery as a profession and they move from town to town practicing their trade. Conan is always asking if anyone has seen the strange serpent symbol. One old man tells them that there is a snake cult of growing power with temples in many towns including this one, but he does not know if they use the symbol he seeks.
Subotai and Conan decide to raid the temple, rumored to have great treasures including a huge ruby called the "Eye of the Serpent." As they prepare to climb the temple's tower they become aware of a woman in the shadows. They confront her finding that she is also a thief, named Valeria (Sandahl Bergman
). They decide to join forces. After climbing the tower Valeria checks out the upper level while Conan and Subotai continue down into a pit containing the "Serpents Eye" guarded by a humongous snake. Fortunately the snake sleeps while the two steal all they can. On the upper level a ceremony is taking place presided over by Rexor. There, a young girl (Leslie Foldvary
) strips and prepares to throw herself into the snake pit to be consumed by the serpent as a sacrifice. Conan is leaving when he sees a medallion on the wall that matches the symbol of Doom's marauders that destroyed his village. He steals the medallion, however, he lingers too long and the snake awakes and attacks. He and Subotai manage to kill it, however, without disturbing the ceremony at the top of the pit. The ceremony reaches its climax and the girl throws herself into the pit. It is only then discovered that the snake is dead. Rexor send guards after the thieves, but they escape.
Over the next days the three thieves celebrate too hard and are easily captured by the King's guard. Brought before King Osric (Max von Sydow
) they expect punishment, but are surprised when the King congratulates them for standing up to the cult. He commissions them to go after his daughter, Yasimina (Valérie Quennessen
), who has been seduced by the cult and travels to meet its leader, Thulsa Doom.
Valeria, who has fallen in love with Conan, does not want to risk their current happiness and advises against going after the princess. She awakens the next morning to find Conan gone. He travels alone to find Thulsa Doom not just to rescue Yasimina, but also to avenge the murders of his parents and clan.
Conan travels to Thulsa Doom's "Mountain of Power" temple in the east. Before he arrives he comes to a sacred place with many standing stones attended by an old wizard, Akiro (Mako Iwamatsu
). They quickly become friends, and the wizard warns Conan of Doom's power.
Conan disguises himself as a follower and goes to the Temple of Set. The temple consists of a massive set of steps up the side of the mountain leading to an ornate entrance into caves that honeycomb the peak. Stealing a priest's robes, he gets as close as the steps before one of the guards recognizes the medallion he carries as being stolen. Thorgrim and Rexor seize Conan, beat him, and drag him before Thulsa Doom, who chides him for stealing the Eye of the Serpent and killing the great snake. Conan replies that Doom killed his parents, but Doom did so much killing as a young man that he does not really remember the incident. He also explains that "steel is strong but flesh is stronger" a point he proves by coaxing one of his female followers to step off a nearby cliff, falling to her death. Doom orders that Conan be crucified on the "Tree of Woe."
Conan is left in the desert, nailed to the ancient tree. He is nearly dead when he is finally rescued by Subotai. Subotai and Valeria take him to Akiro for healing. The wizard tells them that this night the spirits of the underworld will come and try to take Conan's spirit back with them. Valeria says that she will fight them. Akiro warns that the gods will extract a heavy price for defying them in such a manner, but Valeria tells him she is willing to pay it.
They successfully fight the spirits and Conan recovers. Conan finds that the Doom's mountain has cave entrances on the far side connecting with the interior of the temple. The three paint themselves with warrior camouflage and enter the temple through the rear, quietly killing the guards. They work their way to the throne room were an orgy is in progress; the participants are also eating a green stew made of human flesh. They set the drapes on fire and attack creating chaos in the throne room. Rexor recognizes Conan and attacks him with a double-bladed axe, he is soon joined by Thorgrim, wielding a large warhammer. Conan is nearly overpowered, but Thorgrim's warhammer dislodges a pillar, causing some of the ornate masonry to fall on him and Rexor, giving Conan a chance to flee. Valeria grabs Yasimina and they escape. The girl is unwilling to leave, however, and Conan is forced to carry her on his shoulders. He is unable to kill Doom, who has changed himself into a giant snake and escaped the throne room through a small tunnel.
The thieves get to their horses and ride away with the princess. Doom, back in human form, arrives at the rear entrance with Rexor and Thorgrim, swearing revenge. Using an enchanted snake as a poison arrow, Doom manages to wound Valeria in the leg at a great distance. She tells Conan about Akiro's earlier warning of paying the gods, then dies in his arms. Back at the place of the standing stones, Conan burns Valeria's body in a warrior's funeral. Yasimina is tied to a nearby rock and she shouts to Conan that the funeral pyre will be seen by Doom and he will attack. Conan smiles as if he wants Doom to come to him.
At the place of the scared standing stones, Conan, Akiro, and Subotai prepare a defense. They set booby traps around the stones and place defensive spikes. Conan finally prays to his god, Crom, asking him to remember that today two will stand against many, and that if Crom doesn't wish to help Conan, so be it.
Doom comes with his lieutenants and a horde of men. He stands on a distant hillside and watches the battle. Despite being outnumbered Conan and Subotai dispatch most of his troops. Thorgrim is killed by one of Conan's elaborate traps. As he watches Thorgrim die, Rexor, wielding Nial's sword, attacks and nearly kills Conan. He is about to deliver a killing blow when a Valkyrie-like figure intervenes, then asks Conan if he wants to live forever. It is the ghost of Valeria, dressed in shining armor. She has fulfilled a promise she made to Conan that she would come back from the dead, if necessary, to fight by his side. Reinvigorated, Conan finally slays Rexor and reclaims Nial's sword, but the blade has been broken in the battle.
Seeing that his men have lost, Doom decides that if he cannot have Yasimina, nobody will, and prepares another enchanted snake arrow to kill her despite the girl's desperate pleas. Subotai, however, races up the hill and is able to block the snake with his shield at the last moment. Doom rides off, and Yasimina now appears to be free of his indoctrination.
The scene switches to the mountain temple were Doom is conducting a night ceremony. Throngs of his followers stand on the temple steps holding lit torches. Conan appears behind Doom. When Doom sees him he tells Conan that Conan cannot kill him because Doom is the only purpose Conan has in his life. For a moment Conan is taken in by Doom's enchanted voice and hypnotizing stare like his mother was years before, but then breaks free from the spell and beheads Doom with Nial's broken sword. He holds Doom's head up to the crowd, and then tosses it down the steps. Seeing that their leader was not immortal as they thought, the crowd douses their torches in the temple's pool and all quietly depart.
Conan sits on the steps of the temple, alone, thinking about what to do next. He throws a burning oil lamp into the temple, setting it ablaze and returns Yasimina to her father. Akiro speaks, telling us that Conan will have many adventures. The scene switches and we see an older Conan sitting on a throne. A text crawl informs us Conan will one day become a king by his own hand, but that this is a story for another day.