Sakura Kinomoto was an ordinary 4th grader until the day she opened a strange book and let dozens of powerful magic cards loose on the world. Keroberos, the Guardian of the Clow Cards, informs Sakura that it is now her responsibility to find and capture the freed cards. However, much to the reluctant Sakura's dismay, things aren't going to be easy for her. Simply saying magic words and waving her wand around isn't good enough. Each card is a living, thinking, extremely powerful being. She'll have to learn to cope with her new responsibilities, as well as ordinary troubles involving love, school, family, and friends. With the support of her friend Tomoyo and a young boy with powers of his own, she must learn how to use her newly awakened magical abilities to collect each card and prevent the disaster that will befall the world if she doesn't.
Illyasviel von Einzbern is a normal schoolgirl in Japan. She lives with her overachieving nice guy older brother Shirou Emiya and her two maids, her parents are perpetually out of the country, and she has fanciful dreams of using magic to make her dreams of love with Shirou come true. Enter fierce rivals Rin Tohsaka and Luviagelita Edelfelt, two Magi who have been sent to Japan by Zelretch the Wizard Marshall, Master of Parallel Worlds, with the powerful sentient Kaleidosticks to collect the mysterious Class Cards and clean up their mess. After getting fed up with their constant infighting, one Kaleidostick, Magical Ruby, senses Illya's fleeting dream of magic and leaves Rin for her, tricking her into becoming a magical girl. Now Illya is being forced by Rin to collect the dangerous Class Cards in her place. But perhaps more dangerous still are Illya's own mysteries that may have unknowingly attracted Ruby to her.
It has been a month since Illya and Miyu became magical girls and sealed all of the Class Cards. Rin and Luvia have been forced to remain in Japan to clean up the aftermath, such as the now-unstable magic in the area's ley lines. While helping with the cleanup, the seal on Illya's mysterious innate magic goes berserk and she loses control of her transformation. By the time she has realized what happened, a mysterious girl who looks eerily like Illya appears, apparently bent on bringing destruction down on the original whose heart she seems to have spawned from, and making matters worse she holds the power of the Archer Class Card, with which Illya holds a special bond, within her body.