Riko Fukumine, Haruhiko Amami's non-blood-related foster sister, returns to Narasakicho after years away due to family circumstances. Her reappearance disrupts established dynamics, particularly impacting Kirino Konosaka, Haruhiko's childhood friend who harbors feelings for him.

Riko shares a complex history with Haruhiko rooted in their childhood. Before her departure, she violated a pact with Kirino by using Gift magic to create the town's eternal rainbow—a parting gift ensuring Haruhiko would remember her. This childhood promise forbade using Gifts to influence Haruhiko's affections, making Riko's act a source of conflict.

As a high school student distinguished by pink hair and a frequent choker, Riko's return reignites her bond with Haruhiko. Their closeness culminates in his love confession during a school trip, where he rejects Kirino's request for a love charm. Kirino's resentment over the broken pact triggers her retaliation: she deploys a distorted Gift to extract Riko's memories. This erases Riko's recollections of Haruhiko beyond recognizing him as her brother, causing him profound distress.

Haruhiko's investigation into Gift magic leads him to his mother's grave and the origin of Gifts. To restore Riko, he uses his own Gift to erase the eternal rainbow—the source of the town's magic. This action reverses Riko's memory loss, reinstating her identity and past. Following this resolution, Riko and Haruhiko reaffirm their commitment to each other at the location once intended for rainbow viewing.