In an era filled with legends, gods once roamed the Earth, bringing both wonder and torment to humanity. Among these deities was Athena, the Goddess of War and Wisdom, who held the power to extinguish humanity but chose instead to serve as their protector. Over the years, courageous knights from all corners of the world pledged their loyalty to her, mastering the mystical energy known as Cosmo, drawn from the very stars. As time passed, the gods transitioned into folklore, fading from memory. However, 18 years ago, Athena was reborn as a vulnerable child. During her descent to Earth, a valiant Knight donned in golden armor sacrificed his life to shield her. This was the moment Alman Kido (Sean Bean) discovered her, and since that day, he has dedicated himself to training a new generation of knights to serve and protect Athena.
Enter Seiya (Mackenyu), a street-savvy orphan on a relentless quest to find his long-lost sister, Patricia, who vanished many years prior during their childhood. In the midst of a brutal brawl against Cassios (Nick Stahl) in an underground fighting ring, Seiya unwittingly unleashes a mysterious blue light, allowing him to effortlessly defeat Cassios, but he flees the scene before finishing what he started.
After the fight, he encounters Docrates and Kido, who warns Seiya of his ex-wife, Vander Guraad (Famke Janssen), who is searching for him. Kido explains that Guraad manages the fighting ring as a means of recruitment for her private army, specifically targeting those with remarkable skills. Guraad has set her sights on Seiya, seeing in him a powerful warrior whose strong Cosmo could bolster her ranks.
Chaos ensues as Guraad's warriors capture Docrates and hunt down Seiya and Kido, leading to a frantic escape orchestrated by Mylock (Mark Dacascos), Kido's devoted assistant. In the turmoil, Seiya demands answers but is soon knocked unconscious by Mylock, causing memories of his sister's abduction to resurface. After regaining consciousness, he finds himself at Kido's secret hideout.
To Seiya's astonishment, Kido reveals that his adopted daughter, Sienna (Madison Iseman), is the reincarnation of Athena. Seiya is destined to become the Pegasus Knight, a guardian for Athena. Patricia, aware of Seiya's fate, sacrificed herself to keep him safe. Kido suggests that Seiya undergo rigorous training with Marin, the Silver Knight, to harness his Cosmo effectively and to face the threats looming over both him and Sienna.
During a breakfast shared with Sienna, she inadvertently loses control of her Cosmo, prompting Kido to intervene. Sienna recounts her own harrowing backstory; after Kido rescued her as a baby, Guraad had leveraged the fallen Knight's Gold Armor to create technology far beyond human comprehension. She shares with Seiya that mastering their Cosmo could potentially lead him to Patricia.
Meanwhile, Guraad confronts Cassios for failing to capture Seiya. Seeking revenge, Cassios engages in a brutal showdown with Guraad's formidable soldiers, ultimately accepting her offer to join her forces. Guraad's ultimate goal is to find and eliminate Athena before she ascends to full godhood, which would grant her the ability to annihilate the world.
As Seiya and Sienna's bond deepens, she persuades him to train with Marin. Though his initial attempts at training are discouraging, he perseveres under Marin’s guidance. That same night, Sienna experiences a vision where she embodies Athena, unleashing chaos as she destroys Seiya's armor and instigates global devastation.
Seiya gradually masters his Cosmo, awakening his Pegasus armor just in time to confront his past and Kido's involvement in Patricia’s capture. However, complications arise when Sienna reveals her dark history with Guraad, forcing Seiya and Sienna back to Kido's estate. When Guraad's forces invade, chaos erupts, leading to Kido's self-sacrifice to save Seiya and Sienna from certain doom.
Amidst the confusion, Seiya musters a surge of strength to confront Guraad's warriors and ultimately Cassios, awakening fully to his newfound potential. In a dramatic twist, Guraad attempts to drain Sienna’s Cosmo to suppress her transformation into a threat, prompting a profound struggle. As Guraad battles her own inner demons, she inadvertently restores Sienna’s energy just as she becomes a looming threat.
When Sienna awakens, her appearance dramatically shifts, signaling her transformation as a goddess. In a surprising moment of reconciliation, she restores Guraad's lost arms, and together they confront the shifting dynamics of their relationship. With their fates intertwined, Sienna, Seiya, and Mylock embark on a new quest, alerted to the awakening of the old gods and the pressing need to locate Patricia.
