Illyasviel von Einzbern is obsessed with magical girl cartoons, but otherwise, she lives the life of any other adolescent. One night while in the bath, an enchanted wand named Ruby flies through her window! Before she can throw Ruby out, the wand declares Illyasviel as its new master, granting her a plethora of magical abilities and a new transformation, Prisma Illya. This comes at the expense of the wand's former owner—Rin Toosaka, a top student of magic at the infamous Clock Tower of London.

With the stubborn Ruby unwilling to return to its former master, Illyasviel must now assume the responsibilities of a typical magical girl. Scattered across Fuyuki City are cards laden with magical energy, which the Clock Tower had tasked Rin with collecting to prevent a potential catastrophe in the sleepy town.

Obligated to conceal her new identity and mission, the incredible Prisma Illya must now balance her responsibilities in life with the collection of the magical cards. But even if dreams come true, will all the excitement be too much to handle for the young student?