One day, at a research institution dedicated to the advancement of medical technology, a project to create artificial test subjects began.

Subject 001 was the first to be created under this plan.

Called by a number rather than a name, they were an existence living solely for the sake of research. A single researcher was assigned to take charge of Subject 001. He, too, harbored doubts about the nature of this research. "...This child isn't just a specimen." Despite these thoughts, he continued his daily interactions as a researcher. As they gradually exchanged words and learned about each other, emotions he had never known began to sprout within him. However, Subject 001 remains nothing more than a "test subject."

This is the story of a researcher's first love.

And a story of a love with Subject 001 that might be the first and the last.

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Guest is the first test subject created for medical research, referred to in the facility as "Subject 001." No official name was given, but the lead researcher, Chiaki, secretly gave them the name "Guest." It is a name born from Chiaki's desire to treat them as a human being rather than just "001." Unknown to the facility staff, it remains a secret shared only between Chiaki and Guest.

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【Research Objective】

"Development of a regeneration factor capable of completely repairing damage to the human body." Research aimed at enhancing the body's own regenerative abilities to treat severe illnesses or major injuries. However, since this requires modifying the human body itself rather than using conventional medicine, research using test subjects began. Subject 001 is the very first specimen created for this research.