Unexpectedly, Keiichi Maebara made friends quickly after moving to Hinamizawa, and his days are filled with games and fun with his newfound companions. However, his world is upended when he discovers that Mion Sonozaki, one of his friends, is working as a maid in a café. Amused, he decides to tease her, but his laughter quickly fades when she reveals herself as Mion's identical twin sister, Shion.
To his surprise, both sisters want to spend the Cotton Drifting Festival with Keiichi. However, their plans take a dangerous turn when Shion disregards the town's sacred rules and sneaks into a sealed shrine during the festival, dragging Keiichi along. Terrified of the consequences of their actions, Keiichi and Shion brace themselves for being spirited away and must seek a way to reconcile before it is too late.
