Yoshiyuki Otomo operates within a three-member commercial espionage unit investigating lunar corporations. A traumatic fire on Earth cost him his right arm and his lover, Yayoi Okina. This event resulted in an unexplained cybernetic prosthetic arm. Yayoi's final words, hinting at a reunion on the moon, drive his motivations.

During the lunar mission, he encounters Ines, bearing a striking resemblance to Yayoi. Ines reveals a connection to the Sleeper (or Darkness), an ancient entity deep within the moon, and identifies Yoshiyuki as the destined individual to prevent its catastrophic awakening. His cybernetic arm is purportedly the only instrument capable of interacting with the Sleeper's corrosive environment and manipulating its genetic structure.

His investigation uncovers that the Sleeper is an extraterrestrial being monitored by the lunar corporation, and that Ines is one of many clones derived from it, all sharing Yayoi's appearance. Confronted by these clones, some hostile, he grapples with the psychological toll of repeatedly encountering manifestations of his deceased lover. His initial detachment evolves into a more emotionally driven resolve as he reconciles his personal history with the cosmic threat. He utilizes the cybernetic arm to physically dismantle the Sleeper's core DNA strand, fulfilling the prophecy while confronting his loss.

Professionally, he works alongside team leader Shoko Uzuki and colleague Kei Mimura. Their interactions reflect a cooperative but primarily mission-oriented relationship. His personal drive remains anchored in the unresolved grief and mystery surrounding Yayoi's death and her connection to the lunar phenomena.