During the Hyborean Age, a group of sorcerers from Acheron created a mask from the skulls of dead kings and sacrificed their pure-blooded daughters to the dark gods in order to give the mask the power to subjugate the entire world. After killing countless people in their campaign to conquer the planet, the sorcerers were defeated by the Cimmerians led by Corin (Ron Perlman), who destroys the mask, scattering the pieces across the land, and keeping one for himself.
Years later, the Cimmerians are at war with a rival clan. Corin's pregnant wife is fatally injured in the battle, but is able to give birth to a son, Conan, before succumbing to her injuries. Conan's birth on a battlefield, and subsequent survival, is considered to be a powerful omen.
As an adolescent, Conan (Leo Howard) becomes a fierce, but unfocused, warrior. After a young Conan returns from a warrior's trial with the severed heads of members of a savage tribe, Corin takes the boy to his foundry and teaches him the Riddle of Steel. Conan insists that he is ready to wield his own sword, but Corin reminds him that once a blade has been forged, it must still be tempered with ice before it can be wielded. Not long after, village are attacked by armored warriors, sporting a symbol of a large, two headed serpent. The leader, Khalar Zym (Stephen Lang), confronts Corin, demanding his piece of the Mask. When Corin refuses, he is restrained and tortured. Conan, in an angry moment of defiance, attacks one of the warriors, Lucius (Steven O'Donnell), maiming him when he cuts off the man's nose. After recovering the piece of the mask, Zym has Conan and Corin chained to a pole, a cauldron of molten metal hanging precariously above them, then sets fire to the foundry. Corin sacrifices himself to enable Conan to escape. Conan is left an orphan and bears wounds on his palms from his attempts to save his father. Conan wanders out and claims a large sword as his own -- his father's sword was stolen by Zym's daughter, Marique.
Twenty years later, Conan (Jason Momoa) is a mercenary, working with a Zamoran pirate named Artus (Nonso Anozie). The two men raid a slave camp and take the survivors to the city of Messantia. The snake cult has grown in size over the last decade, and they have a small garrison in the city. Conan sees Lucius, now an out-of-shape warden, pursuing a pickpocket, Ela-Shan (Saïd Taghmaoui), and allows himself to be arrested in a bid to learn more about his enemy. Conan soon escapes prison and tortures Lucius for information about Zym and the snake cult. After Conan promises to spare him, Lucius reveals that Zym is seeking the pure blood descendant of the sorcerers of Acheron, as it will take their blood to unlock the Mask's true power. Satisfied that he has gotten all he can out of Lucius, Conan leaves him at the mercy of the liberated prisoners, reminding the frightened guard "I only promised I wouldn't kill you." Ela-Shan tells Conan that, if he ever needs him to return the favor, Conan will find him at Arlagon, the City of Thieves.
Meanwhile, Zym and Marique (Rose McGowan), attack a monastery where they believe the Pureblood is. Sensing something is wrong, the elder priest Fassir (Raad Rawi) instructs one of his students, Tamara Amalia Jorvi-Karashan (Rachel Nichols), to flee to her birthplace, then confronts Zym to buy time for her to escape. Zym recounts how his wife, Maliva, was murdered by Fassir's order. Fassir defends this decision, insisting that Maliva was an insane fanatic who was attempting to unleash occult forces to destroy Hyborea. Enraged by Fassir's defiance, Zym kills him by smashing his head against the marble steps. Marique tests the blood of the remaining students and realizes that the Pureblood is not among them.
Tamara's carriage comes under attack by snake cult warriors. Conan comes to her rescue, after a spirited chase, eventually captures their captain, Remo (Milton Welsh). After forcing Remo to reveal Tamara's importance, Conan kills him by catapulting him into Zym's camp with a message in his mouth for Zym to meet him at the Shaipur Outpost alone. Marique verifies that the cloth the message is written on belongs to Tamara.
Confident that Zym will not recognize him, Conan masquerades as a bounty hunter, offering to sell Tamara to Zym for a substantial sum of gold. Once he is satisfied that Zym and Marique have come alone, Conan reveals his true intentions, and challenges Zym to a sword fight. At her father's instructions, Marique engages Conan with soldiers made of sand before Zym battles Conan. Conan and Zym first seem to be nearly evenly matched, but Conan's rage gives him the upper hand. Marique ultimately swings the fight in her father's favor by poisoning Conan with a poison-laced boomerang dagger. Tamara rescues him and they return to Artus' boat, stationed nearby, where Artus helps Conan recover.The boat is attacked by the snake cult, who kill several of the crew, though are ultimately repulsed. Conan orders Artus to return to Messantia with Tamara and departs to confront Zym in his kingdom. Artus tells Tamara that Conan left a map behind and she follows him, meeting with him in a cave, where they make love. The next day, as she's returning to the boat to join Artus so they can sail away, she's captured by the snake cult.
Remembering Ela-Shan's earlier promise, Conan travels to Argalon and hires Shan to help him sneak into Zym's fortress. They discover that beneath the fortress is an old mine, and this is where Zym and Marique plan to sacrifice Tamara, Conan enters through an undersea cave, where the snake cult are preparing to feed prisoners to a giant multi-tendriled serpent, the Dweller. After killing the beast, Conan proceeds further down the mine. Conan attacks the snake cult before they can complete the sacrifice, but Zym has collected enough of Tamara's blood to restore the Mask to suboptimal strength. Zym dons the mask, claiming to be invincible, and proceeds to fight Conan again. Zym uses Corin's sword to fight Conan, but Conan is eventually able to wrest it from his opponent. The battle inadvertently triggers an earthquake, killing most of Zym's followers and rendering the mine unstable. Conan frees Tamara and tells her to run. However, she is followed by Marique. Conan hears Tamara's screams, and kills Marique by throwing her down a mine shaft where she's impaled on a post. As the mine continues to crumble, Conan and Tamara attempt to find an alternate way way out, with an enraged Zym in hot pursuit. Conan and Tamara eventually make their way to an unstable rope bridge, precariously above a massive canyon. As they cross the bridge, a support beam under Tamara gives way, causing her to nearly fall. As Conan attempts to save her before the bridge gives way, they are cornered by Zym, who uses the Mask's power to summon Maliva's spirit. In an act of defiance, Conan destroys the bridge supports with his sword, before jumping to safety with Tamara. The bridge collapses, and Zym falls to his death.
With the snake cult destroyed, Conan returns Tamara to her birthplace, telling her that they'll meet again. He then returns to his old village and tells his father that he has avenged his death and recovered the sword Zym stole from him, before departing in search of new adventures.