On a rainy day Ginko takes a rest from his travels and comes across a man named Kourou who seems to be obsessed with a rainbow. To pass the time Kourou reminisces in his past, with the events which led to him pursuing rainbows whenever they appear. Ginko explains to him that the rainbow which he is searching for is actually a Mushi known as "Koda" but there seems to be more to it.

Some kids enter Adashino-sensei's shed and find a lot of strange things, but there's one thing in particular that interests them. It's night and Adashino-sensei is called upon to look at one of the kids, a girl who has become bitterly cold, and the cause is a mysterious ink stone she touched. Luckily for Adashino-sensei, Ginko is passing through the village, and he shows him the ink stone, which he identifies it as a "Kumohami."