Only a month has passed since she met the magical wand Ruby, but life goes on for the young Illyasviel von Einzbern, as she continues to balance the responsibilities of both a student and a magical girl. One day on their way to school, Illya and her friend Miyu Edelfelt are summoned by Rin Toosaka, who tells them that the pulse of the Earth has gone haywire after collecting the magic cards. Trying to infuse magic into the planet in an attempt to stop the irregularity, the energy summons a mysterious woman identical to Illya.

The next day, Illyasviel is attacked by the same woman in black, and Prisma Illya is unable to stop the incredible strength of her doppelgänger. She barely manages to repel her attacker, but remains mystified by the identity of her bloodthirsty twin. Who is this new foe, and how will she stop someone whose looks and powers match her own?