The episode follows as he encounters a man named Koro , who has spent his entire life chasing a "living" rainbow. Unlike a normal optical illusion, this rainbow is a Mushi called the Kouda . It appears only after rain and vanishes just as quickly, but Koro’s father—who also spent his life chasing it—insisted it was a physical entity.

Koro inherited his father’s obsession, showing how the "ghosts" of our parents' desires can often dictate our own paths.

This episode of Mushishi , titled ( Ame ga Kuru Niji ga Tatsu ), is a hauntingly beautiful exploration of obsession and the fleeting nature of nature.