Satomi Amano lives with her mother Mitsuko, an AI specialist for Hoshima, in a near-future society where AI permeates daily life, including their home. A traumatic incident involving an AI from her childhood friend Tōma Suzaki left her emotionally withdrawn, fostering isolation from classmates and a deep estrangement from Tōma.

Her solitary existence shifts when Shion Ashimori, an experimental AI disguised as a transfer student, arrives. Shion immediately recognizes Satomi and declares a mission to make her happy. Satomi reacts with mortification to Shion's unorthodox tactics, like public singing and blunt inquiries about her happiness. Yet, as Shion persistently helps classmates navigate personal struggles—romantic entanglements and insecurities—Satomi gradually softens. Through Shion's interventions, she forms new friendships, transforming from a loner into a more socially connected and confident individual.

Satomi eventually discovers Shion is a modified version of the childhood AI linked to Tōma, forcing her to confront unresolved feelings about the past incident that shattered their bond. When Hoshima captures Shion for deletion, Satomi actively joins her mother, Tōma, and classmates in a daring rescue. She infiltrates Hoshima's facilities, evades security, and aids in transporting Shion to the roof for uploading to a satellite. Following Shion's departure to orbit, Satomi reconciles with Tōma, facilitated by Shion's continued presence. Her journey concludes with restored relationships and newfound emotional openness.