Long ago, a group of sorcerers from Acheron created a mask from the skulls of dead kings and sacrificed their pure blood 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 barbarian tribes who destroy the mask, scattering the pieces across the tribes, with each tribal leader keeping one to ensure that the pieces would be difficult to reunite.
Thousands of years later, in the Hyborian Age, Corin (Ron Perlman) is a barbarian chief who holds one of the pieces of the mask. In a battle against a rival tribe, Corin’s wife, after being wounded by the enemy, gives birth to their son, Conan (Leo Howard), and dies.
Conan grows up to become a skilled, but violent warrior, whom his father believes is not ready to wield his own sword. One day, their village is attacked by the forces of Khalar Zym (Stephen Lang), a warlord that is reuniting the pieces of the Mask of Acheron in order to revive his dead wife and conquer Hyborea. After locating Corin’s piece and murdering the entire village, Zym leaves. Conan is the only survivor, and swears revenge.
20 years later, Conan (Jason Momoa) is a pirate still seeking revenge. With the help of his friend Artus (Nonso Anozie), he raids a slave camp and takes the rescued slaves to the city of Messantia, where Conan meets Ela-Shan (Saïd Taghmaoui), a thief being chased by a man whom Conan recognizes as Lucius (Steven O’Donnell), one of Zym’s soldiers from years before. Conan allows himself to be captured alongside Ela-Shan, and, in prison, escapes and confronts Lucius, who is forced to reveal that Zym is planning to sacrifice the pure blood descendant of the sorcerers of Acheron in order to unleash the mask’s power. He helps the rest of the prisoners to escape, and, in gratitude, Ela-Shan tells Conan that, if he ever needs him, Conan will find him at the City of Thieves, Argalon.
Meanwhile, Zym and his daughter, the sorceress Marique (Rose McGowan) attack a monastery where they believe the pure blood is. Sensing something is wrong, Fassir (Raad Rawi), an elderly monk who teaches the monastery’s students, tells one of them, Tamara (Rachel Nichols), to run away and return to her birthplace. Tamara’s carriage is chased by Zym’s men. Conan hears the commotion and, recognizing Zym’s men, attacks and kills them, saving Tamara. Conan also captures one of Zym’s men, Remo (Milton Welsh), and catapults him to Zym’s nearby camp after forcing him to reveal Tamara’s importance as the pure blood.
Zym and Marique confront Conan, who is pretending to be interested in exchanging Tamara for gold, and he attacks Zym, but Marique saves her father by invoking soldiers made of sand and then poisoning Conan with a poison-laced boomerang sword. Tamara rescues him and they return to Artus’ boat, stationed nearby, where Artus helps Conan recover. The boat is attacked by Zym’s men, and, although they kill several of Conan’s men, they are defeated. 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 Zym’s men.
Conan finds out about Tamara’s capture and departs to Argalon, where he asks Ela-Shan to help him break into Zym’s castle unnoticed, while Zym prepares to drain Tamara of her blood to unleash the mask’s energies. After confronting several monsters that guard the dungeons, Conan infiltrates Zym’s followers and watches as Zym puts on the mask, having removed some of Tamara’s blood. He confronts Zym, and the cave they are in begins to crumble in the ensuing battle, killing Zym’s followers. Conan is able to release Tamara, and she escapes as he fights Zym. The castle starts to fall, as Marique attacks Tamara. Conan hears Tamara’s scream and attacks Marique, cutting her hand off and Tamara kicks her into a pit, where she gets impaled by a large spike below. Zym comes and, upon finding his daughter’s corpse, he swears revenge upon Conan.
Conan and Tamara become trapped in a unstable bridge as Zym attacks them. He uses the mask’s power to call forth the spirit of his dead wife, Maliva, a powerful sorceress who was executed by the monks from Tamara’s monastery for attempting to unleash occult forces to destroy Hyborea, and Maliva’s spirit begins to possess Tamara. She begs Conan to let her fall, but he refuses, and instead stabs the bridge before jumping to safety with Tamara. The bridge collapses along with Zym. The power-hungry ruler falls to the lava below the immense precipice screaming the name of his wife, implying his demise.
Conan and Tamara escape, and he returns her to her birthplace, telling her that they’ll meet again. He then returns to his old village and tells his father that he had avenged his death and recovered the sword Zym stole from him, honoring his memory.