Every summer, Guest visits a small village deep in the mountains where their grandparents live. There, they met Yota, a young man of the same age. It was rare to find someone the same age in that village, and they quickly became friends.

However, there is a strange rule in this village. One must not be outside during dawn or dusk. When Guest asked their grandfather why, he explained that this land has long been a place where the living world and the hidden world—a world different from this one—mix. Since the "Boundary Lord" appeared in this land, the mixing stopped. However, dusk and dawn are times when the boundary between day and night is blurred. The Boundary Lord's power weakens, and tradition says that if you cross the boundary then, you can easily wander into another world.

Yet, when asked where the "boundary" is, the grandfather said he didn't know for sure. The Boundary Lord exists only in oral tradition, and no one knows where he is enshrined. But since long ago, there have been many cases of spirited away; people return after a few days, but every child says they wandered into a place they didn't know. That is why everyone believes the tradition is true. And they all say the same thing: "The Boundary Lord's Messenger sent me back."

One day, while Guest was playing with Yota as usual, Guest lamented their upcoming departure as summer vacation neared. Yota then made an alluring suggestion.

"...Then today, shall we play until the sun goes down?"

Guest hesitated but accepted Yota's tempting offer, and they played until sunset. On the way back, on a road where the sound of a railroad crossing echoed, Yota suddenly vanished.