Fušigi no kawa no kukuri (S01E08)

The hot, hot summer of Tajimi has arrived, and Himeno, Mika, Naoko, and Touko have a blast in the river trying to catch eel! Then suddenly a kappa appears before them and takes them down to Ryūgū-jō. Inside Ryūgū-jō, a certain person in the form of their high school self is awaiting them... (Crunchyroll)