The first installment of the Neptune OVA Series Trilogy.
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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.
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(Source: Crunchyroll)

Choujigen Game Neptune THE ANIMATION: Nepu no Natsuyasumi