Kotona Takashina, a popular and innocent Tokyo high school student, shares a romantic bond with Haruto Ichihara and a close friendship with Yusuke Ninomiya. Her soulmate is Princess Astrid from another world, united by their mutual fondness for sour oranges.

While returning from a date with Haruto, Kotona is stalked by a Black Banner Assassin who fatally stabs her with a dagger. Yusuke arrives first at the scene, followed by Haruto attempting urgent medical aid. Amidst the chaos, both boys vanish into another world, though Kotona’s immediate rescue details remain ambiguous.

Upon her friends’ return, doctors diagnose Kotona with a malignant tumor, granting her three months to live. Her health declines swiftly, necessitating sedation. This tumor is mysteriously linked to a curse afflicting Princess Astrid. After Galeroth Felgrimm’s defeat in the other world, Kotona’s condition dramatically improves, allowing successful surgical removal of the tumor.

A ripple effect erases Kotona’s memories of Yusuke, mirroring his broader disappearance from her world’s collective consciousness. Despite this loss, she rebuilds her life happily alongside Haruto.