To escape the summer heat, Mashiro and Momiji are enjoying the cool air near a waterfall, not far from Kirinomori Shrine. Then, they receive a call from their grandfather. Apparently, Kirinomori Shrine has received a request.

It seems that yokai (monsters) have started appearing in the town near an island shrine. Because of this, the summer festival the islanders look forward to might be cancelled. The two are asked by the chief priest to exorcise the yokai, and they readily accept the request.

Mashiro and Momiji head to the island to investigate, in order to save the island and ensure the summer festival takes place successfully.