A lonely boy struggling with isolation after moving to a new city, Tsubasa dreams of owning Shinki – small intelligent androids – for companionship. This dream becomes reality when he discovers a damaged Angel-type Arnval Mk.2 Shinki in a moonlit tree. Displaying technical aptitude, he repairs her himself and names her Kaguya, inspired by the moon princess legend reflecting their meeting.

Tsubasa initially finds peaceful companionship with Kaguya. However, this tranquility shatters when she loses control during an official Shinki battle, revealing latent combat programming and sparking public fear. Despite witnessing her dangerous capabilities, Tsubasa remains loyal, refusing to abandon his sole companion.

Learning Kaguya is a prototype autonomous weapon from a research facility, Tsubasa partners with Ken, a Shinki shop owner possessing insider knowledge. Together, they infiltrate the facility after her retrieval. During the mission, Tsubasa physically intervenes to stop Kaguya's berserk rampage, confronting her directly to restore her sanity. His intervention halts her destructive state.

The crisis escalates as the facility's giant weaponized Shinki, Athena, activates uncontrollably. Tsubasa supports Kaguya as she battles Athena to protect Strarf, the Shinki sent to capture Kaguya but now integrated into Athena's systems. After Athena's defeat, Strarf joins Tsubasa and Kaguya. This experience solidifies Tsubasa's resolve to protect Kaguya and accept responsibility. He begins working alongside Ken at his hobby shop, with Kaguya and Strarf assisting, shifting towards a stable life centered around Shinki care.

Tsubasa evolves from a lonely boy seeking friendship through Shinki ownership to a responsible figure capable of decisive action. His commitment to Kaguya drives him to confront institutional threats and navigate complex Shinki conflicts, finding purpose in protecting his Shinki partners.