Waking up to find herself in a parallel version of Fuyuki City, Illyasviel "Illya" von Einzbern is lost and alone. She discovers her home in ruins, with a massive crater lying in the center of her hometown. With snow falling in the middle of summer, confusion consumes the young elementary schooler, who has no knowledge of where her friends or her wand Ruby may be.

Making it to the remains of her house, she is suddenly tackled by an amnesiac girl. Wearing a gym uniform as the icy temperature sets in, the mysterious girl has no idea of where she is or why she showed up. However, this stranger, known as Tanaka, apparently has information about the location of Rin Toosaka, Miyu Edelfelt, and the rest of Illya's missing friends.

Fleeing from agents of the Ainsworth family⁠—those in control of this parallel realm—where will these two end up, and how will Illya restore the present back to the world she once knew?