In the remote mountain village of Okukozome-chou, siblings Kasugano Haruka and Sora recall cherished memories of summer visits to their grandfather's home. Following a tragic loss, they relocate to their grandfather's place where they reunite with old friend Yorihime Nao, classmate Nakazato Ryouhei, shrine maiden Amatsume Akira, and Kazuha, a friend of Akira. Amidst the serene surroundings lies a hidden pact between the twins, a valuable missing item, and the true purpose behind their return, shrouded in mystery.