The protagonist, Eiya, a lone wolf thief known as "Black Eagle Eiya," finally reaches the lowest level of an ancient, trap-filled dungeon after 26 years of effort. His only skill is "good eyesight," which he uses to navigate by stealth and observation rather than direct combat. Upon activating a complex mechanism, his memory abruptly cuts off.
Eiya awakens in complete darkness to a disembodied, cheerful, and incredibly annoying voice. He finds himself on a hard stone floor, feeling disoriented. The voice, which claims to have no solid sense of self but knows Eiya's thoughts, reveals itself to be the "treasure" Eiya sought, a being sealed within the dungeon. This being, who calls itself a "life-saver," explains that Eiya's activation of the mechanism triggered a self-destruct sequence, causing the dungeon to collapse and Eiya to be buried alive. The being's seal was also partially broken, allowing it to communicate.
The being reveals it has been waiting for Eiya for centuries, manipulating events to bring him to this point. Eiya, furious at having his life controlled and now facing death due to this entity, initially refuses to cooperate. However, the being, which identifies itself as a "monster" but possesses immense power and a profound lack of understanding of human morality, pleads for Eiya's help to fully break its seal. It proposes a contract, asking Eiya to name it.
After much hesitation and internal conflict, Eiya agrees to the contract, driven by a desire to survive and a sense of responsibility. He names the being "Viede," meaning "empty," reflecting its lack of self-identity and understanding. Viede, overjoyed at receiving a true name and the prospect of freedom, reveals its ability to manipulate fate.
The contract allows Viede to escape the collapsing dungeon with Eiya. They are transported back to the moment before Eiya triggered the mechanism. Eiya, now aware of the danger, guides Viede through the dungeon, relying on his knowledge of the traps and Viede's surprising agility and stamina, despite her seemingly delicate appearance. Viede's lack of understanding of human limitations and the practicalities of survival becomes apparent, leading to a near-fatal incident where Eiya must save her from a collapsing trap.
Upon escaping the dungeon, Eiya realizes the immense responsibility that comes with Viede's power. He teaches Viede about the importance of effort, the nuances of human interaction, and the need to live life with purpose, even within the constraints of fate. Viede, in turn, expresses a desire to understand humans better and asks Eiya to guide her.
Their immediate plan is to return to civilization and celebrate their survival. Viede, a being of immense power and beauty, struggles with her own existence and the implications of her abilities. Eiya, a cynical but ultimately principled thief, finds himself entangled with this powerful, naive entity, setting the stage for future adventures and challenges as they navigate the world together.
