Toosaka Rin and Luviagelita Edelfelt are magi from the Clock Tower, the headquarters of the Mage's Association, and they were sent by its director to collect Class Cards, each holding the powers of a Servant from the Holy Grail War. To aid in their quest, they were handed two powerful magic wands, Rin receiving Magical Ruby and Luvia taking Magical Sapphire.
However, due to the two girls' inability to get along, the wands got fed up with them and broke their contract. Ruby then sought a new master, and found one in an unsuspecting school girl by the name of Illyasviel von Einzbern. Despite Illya's protests, Ruby insisted in making a magical girl out of her, and she is now Prisma Illya. With no knowledge in magecraft and no previous training of her own, being in fact a completely normal young girl with no outstanding abilities, now she must collect the powerful Class Cards using whatever limited aid Rin can give her. Will the two at least to enter a truce with Luvia and Sapphire's new master and work together for this goal?