It is the middle of October, and high school student Tsuruta is reluctantly searching for his sister's lost cat, Luna. When he spots the feline by a tunnel, Tsuruta chases it through the passage. But when he emerges on the other side, he finds his world quite different from before. It's now August again, and standing before him is his nihilistic friend Kamei who had passed away that summer. Unsure of what is real, Tsuruta enlists Kamei to help him in his search. As the two look for Luna, the past and present collide as Tsuruta's unspoken regrets begin to bubble to the surface.