"Absolute Duo" is set in a world where individuals possess unique weapons called Blaze, manifestations of their souls. The story centers on Tooru Kokonoe, a student at Koryo Academy, an institution training individuals to become skilled warriors known as Blazers. Each student is paired with a partner to form a Duo, enhancing their combat abilities through synchronization. Tooru's Blaze takes an unusual form—a shield instead of a conventional weapon—marking him as an outlier among his peers.

Upon arriving at the academy, Tooru is paired with Julie Sigtuna, a silver-haired girl of Nordic descent who wields dual swords. Their partnership is initially strained due to Julie's cold demeanor and Tooru's struggle to adapt to his unconventional Blaze. However, as they face various challenges and adversaries, their bond strengthens, and they learn to complement each other's strengths and weaknesses. The narrative explores their growth as individuals and as a team, delving into themes of trust, resilience, and self-discovery.

The academy serves as a backdrop for both training and conflict, with students engaging in battles to hone their skills and protect their world from external threats. Tooru and Julie encounter other students, each with their own motivations and struggles, including the enigmatic Miyabi Hotaka and the cheerful Tomoe Tachibana. These characters contribute to the broader narrative, adding layers of interpersonal dynamics and rivalries.

As the story progresses, darker elements emerge, revealing conspiracies and hidden agendas within the academy. Tooru and Julie uncover secrets about the nature of Blazes and the true purpose of their training, forcing them to confront moral dilemmas and the consequences of their actions. The series balances action-packed combat sequences with moments of introspection, exploring the emotional and psychological toll of their journey.

"Absolute Duo" combines elements of action, fantasy, and school life, presenting a narrative that intertwines personal growth with larger, overarching conflicts. The characters' development is central to the story, as they navigate the complexities of their relationships and the challenges posed by their world. The series maintains a focus on the interplay between individual identity and collective responsibility, creating a dynamic and engaging narrative.