Yuuto's older sister disappeared seven years ago after entering a peculiar door, leaving only a necklace in her wake. Recently, he caught a glimpse of the same door and observed two unfamiliar girls, Saya and Hina, slip through it. Hoping to uncover any clue regarding his sister's vanishing, he and his feline companion pursued them through the door. Yet, he soon discovered the reason behind his sister's reluctance to bring him along. His naivete plunged him into catastrophe in the dangerous realm beyond the door. Saya now compels him to revisit the door to rectify the loss of life that occurred during his initial journey.
