Young Red Sonja's homeland, Hyrkania, faces a devastating invasion from barbaric forces that brutally thwarts her people, forcing her to escape into the expansive woods surrounding her village. In the solitude of the forest, Sonja dedicates her time to searching for any other survivors from Hyrkania while paying homage to the forest goddess, Ashera.
Years later, a group of mercenaries infiltrates the forest with the dark intention of hunting wildlife for their horns. Intrigued, Sonja tracks these hunters and discovers that they are capturing exotic creatures to participate in gladiatorial games held in honor of Emperor Dragan. Upon Dragan's arrival to inspect the captured animals, Sonja decides to take a stand against their cruelty, launching an attack on the hunters. Unfortunately, her uprising attracts the attention of Dragan's guards, and she finds herself overpowered by the ruthless General Karlak. As a consequence of her actions, Dragan orders Sonja to be taken to the capital, where she is forced to fight in the games.
In the capital, Dragan possesses the first half of an ancient Hyrkanian book that bestows him with profound scientific knowledge, allowing him to harness extraordinary arcane power. His ambition drives him to seek the missing half of the text, believed to be owned by exiled Hyrkanian refugees. He thinks that Sonja may possess a map to their whereabouts, which prompts his interest in capturing her.
As the gladiator games begin, Sonja finds herself pitted against a formidable cyclops controlled by one of Dragan's fiendish devices. With courage, she manages to sever the device's control and turns the cyclops against its captor, causing injury to Dragan himself. Seizing the moment, Sonja destroys Dragan's energy source and manages to escape into the mountains with a group of fellow captives while being pursued by Dragan’s forces, including Annisia, a fierce mercenary afflicted by schizophrenia. Dragan, recognizing Annisia’s skills, employs her promising her hand in marriage in exchange for Sonja’s capture.
In an effort to divert Dragan's focus away from her allies, Sonja attempts to work solo; however, her comrades insist on sticking together. They devise a plan to raid a town that supplies creatures to Dragan, hoping to draw him out. Just then, they encounter the exiled Hyrkanians, who reveal they possess the second half of the coveted holy book that Dragan seeks.
As the epic confrontation unfolds, Dragan arrives with his troops, igniting chaos. While the battle wages on, Dragan’s forces ultimately crush the Hyrkanian resistance, allowing him to reclaim the second half of the book. In an intense fight, Sonja and Annisia engage in combat, and just as Sonja is gravely injured, her loyal horse intervenes, knocking Annisia away. It is then that Sonja is brought before the statue of Ashera, where she experiences a profound vision of her deceased mother, who conveys a message of strength and healing.
Revitalized and healed by Ashera’s grace, Sonja joins her comrades again in a final assault on Dragan. In a tragic revelation, Annisia confronts Dragan only to find out that his harmful treatments caused her mental anguish. In a bittersweet confrontation that turns into a lovers' quarrel, both Annisia and Dragan fatally wound each other, leaving Dragan to escape into the wilderness.
Sonja tracks Dragan down and confronts him as he lies dying. In their final moments together, Dragan dismisses the value of the book, believing its focus on healing and nature is insignificant compared to power. Sonja reveals her connection to him, confessing that she holds herself accountable for his dark transformation, stemming from the tragedy that ripped them apart. As Dragan dies, he clutches her hand, and the pages of the sacred book scatter into the wilds.
With the chapter of her homeland behind her, Sonja ventures out into the world, setting off on new paths of discovery. In a reflective epilogue that takes place years later, one of her fellow fighters finds her in a tavern, urgently calling for her aid. Sonja sharpens her weapons, preparing for yet another adventure that awaits her.
