It is based off of the ero-game entitled Gift (ギフト).

Gift is a story that revolves around high school student Haruhiko Amami and his strong connection to the mysterious power that surrounds the town of Narasaki. It has two distinguishing characteristics: a rainbow that seems permanently fixed in the sky no matter the weather, and an ability known as Gift possessed by all its inhabitants. Gift's power can only be evoked by each person once in a lifetime, and is able to grant a single miracle. To be successful, a Gift must reflect the shared feelings of both the giver and the receiver. If the feelings are not mutual, the Gift is distorted and has the ability to wreak havoc on the surrounding area. When this happens, a black streak appears in the rainbow above town until Gift is used correctly. The story begins as Haruhiko's younger sister, Riko Fukamine, returns home after several years away. Beginning with her appearance, Haruhiko's life starts to change very quickly and feelings of the past begin to surface once more.