"Saint Seiya: The Lost Canvas" is a prequel to the original "Saint Seiya" series, set in the 18th century during the Holy War between the goddess Athena and the god of the underworld, Hades. The narrative centers on Tenma, a young boy from Italy who becomes the Pegasus Saint, and his childhood friend Alone, who is revealed to be the reincarnation of Hades. The story explores their intertwined fates as they find themselves on opposing sides of the conflict.

The series begins with Tenma training under the guidance of Dohko and Shion, the future Gold Saints of Libra and Aries, respectively. Alone, a kind-hearted artist, is manipulated by Pandora, a servant of Hades, into awakening his true identity as the god of the underworld. As Hades's influence grows, Athena's Saints must unite to protect humanity and prevent the destruction of the world. The Lost Canvas, a mystical realm created by Hades, serves as the battleground where the final confrontation between the forces of Athena and Hades unfolds.

Key characters include Tenma, whose determination and growth as the Pegasus Saint drive much of the narrative. Alone, torn between his human emotions and his role as Hades, adds a tragic dimension to the story. Other notable figures include Sasha, revealed to be the reincarnation of Athena, who struggles to balance her divine duties with her personal connections, and the Gold Saints, each representing one of the twelve zodiac constellations, who play pivotal roles in the war. The series delves into themes of friendship, loyalty, and the burden of destiny, as the characters face moral dilemmas and personal sacrifices in their quest to protect the world from Hades's wrath.

The plot is structured around the escalating conflict, with battles between the Saints and Hades's Specters forming the core of the action. The narrative builds toward a climactic showdown in the Lost Canvas, where the true extent of Hades's power and the sacrifices required to defeat him are revealed. The series combines intense combat sequences with emotional character arcs, creating a balance between action and drama. Through its exploration of the Holy War and the characters' struggles, "Saint Seiya: The Lost Canvas" expands the lore of the "Saint Seiya" universe while maintaining a focus on the human elements of its story.