The first installment of the Neptune OVA Series Trilogy.

Neptune and her friends visit "Watashi no Natsuyasumi Yama" (My Summer Vacation Mountain) filled with nature to enjoy their summer vacation. The four goddesses, who gather after a long time, are having a good time as usual. During the vacation, Nepgear and other sisters go to the mountains to wander around. After going through the stretch of a long torii (Shinto shrine archway), the sisters wander off into a different dimension. Then Nepgear meets a woman who just looks like her sister Neptune in her adult form.

(Source: Crunchyroll)