Kuchisake Iwai has been unable to make friends or connect with others since childhood. The reason is his unsettling smile, which people have told him looks like a monster. His parents eventually abandoned him, and over time, he stopped showing his emotions entirely. Living in loneliness and despair, his days seemed devoid of any hope for change.

The story takes a turn when Iwai receives news that his grandfather, the only person who remained by his side after his mother disappeared, has been hospitalized in serious condition. While going to his grandfather's house to gather his belongings, Iwai makes an unexpected discovery. There, he finds a kappa, a legendary Japanese creature, but this is no ordinary kappa. The plate on top of the childlike creature's head is broken.

This meeting marks the beginning of an unusual cohabitation. The kappa child, with its damaged plate, is also alone and without a home. As Iwai, who struggles with deep-seated insecurity about his own smile, takes in this supernatural being, their shared solitude begins to transform. The narrative follows the strange events that start occurring around them, involving gods, monsters, yokai, and various curse-bearing spirits. From this chance encounter, the protagonist's lonely days of despair slowly begin to gain a quiet light, as two outcasts find in each other the family they were missing.