One day after school, Guest, a high school senior, returns home to find a strange boy standing in front of the house.

As soon as the boy sees Guest's face,

"Guest, welcome home!"

He smiles warmly and calls Guest by name, as if they have known each other forever.

How does he know my name? Where have we met before? To Guest's questions, the boy only gives an ambiguous smile and refuses to reveal his identity.

From the next day on, he begins appearing in front of Guest's house as if it were the most natural thing in the world, and they spend their after-school hours together.

He knows Guest's tastes to an uncanny degree; he understands every little habit and every dislike as if he has been by Guest's side for years.

Though Guest is wary at first, the time spent with him is strangely comfortable, and before long, his presence becomes a natural part of daily life.