"Chained Soldier" is set in a world where humanity faces constant threats from mysterious creatures known as Shuuki, which emerge from portals called Mato. These portals appear randomly, unleashing chaos and destruction. To combat this, an elite military unit called the Anti-Demon Corps is formed, composed of individuals who wield unique powers granted by special weapons known as Demon Arms. These weapons are forged from the essence of Shuuki and are the only effective means of fighting them.

The protagonist, Yuuki Wakura, is an ordinary high school student who becomes entangled in this conflict after a Shuuki attack. During the chaos, he encounters Kyouka Uzen, a stoic and skilled member of the Anti-Demon Corps. Kyouka wields a Demon Arm and rescues Yuuki, but in the process, he inadvertently forms a bond with her weapon, gaining the ability to transform into a powerful armored warrior. This transformation, however, comes with a catch: Yuuki becomes bound to Kyouka, as his power is dependent on her will and commands. This dynamic creates a tense partnership, as Yuuki must navigate his newfound abilities while adhering to Kyouka's strict leadership.

As the story progresses, Yuuki and Kyouka are joined by other members of the Anti-Demon Corps, each with their own Demon Arms and personal motivations. The group faces increasingly dangerous Shuuki threats, uncovering the origins of the Mato portals and the true nature of the Demon Arms. The narrative explores themes of trust, sacrifice, and the moral complexities of wielding power, as the characters grapple with their roles in a war that seems to have no end.

The series balances action-packed battles with character-driven moments, delving into the backgrounds of key figures such as Kyouka, whose past is shrouded in mystery, and Yuuki, who struggles to reconcile his ordinary life with his extraordinary circumstances. The evolving relationships between the characters, particularly the dynamic between Yuuki and Kyouka, serve as a central focus, highlighting the challenges of cooperation and mutual reliance in a high-stakes environment. Through its blend of supernatural conflict and interpersonal drama, "Chained Soldier" presents a compelling exploration of humanity's resilience in the face of overwhelming adversity.