The initial segment of the Neptune OVA Series Trilogy centers around Neptune and her companions' trip to "Watashi no Natsuyasumi Yama," a mountain teeming with natural scenery, for their summer vacation. The four goddesses, reunited after a prolonged absence, relish the opportunity to catch up and enjoy each other's company. As their vacation progresses, Nepgear and her siblings venture to the mountains to explore. However, once they pass through a lengthy torii (Shinto shrine archway), they become transported to an alternate dimension. There, Nepgear encounters a woman who bears an uncanny resemblance to her sister Neptune, albeit in her adult form.