The Clow Cards are magic cards created by the ancient sorcerer Clow Reed. Each cards represents different magical properties and each of them is extremely powerful, when they are without a master, will do things on their own, which means they can also be dangerous if not controlled. The Cards have some intelligence and the occasional card has a "personality". Some cards are friendly and playful, and others are mischievous. Some of the more powerful cards are dangerous. A few cards are so intelligent that they can talk and reason.Thus, Clow Cards were the result of a combination of both Eastern and Western magic that formed the basis of an entirely new type of wizardry unlike anything else seen at the time. Since the cards were powerful enough to destroy the world, Clow Reed sealed the cards in a book called the Clow Book. He designed the book so that it could only be released by someone possessing magical powers.In order to protect the world and as a precaution in case the Cards escape, Clow created two magical beings, Yue and Cerberus. Cerberus represents the Guardian Beast of the Seal and rules the Sun. He is to become the Cardcaptor's valued mentor and teacher. Yue represents the Judge of the Cards and rules the Moon. He has the role of determining whether the person who come into possession of the Cards are worthy of them.
When Sakura opens the book and accidentally releases the magical Clow Cards. Cerberus, awakens and emerges from the book's cover. Upon learning the cards are gone, he tells Sakura that she must have special powers, and that it is now her responsibility to retrieve the missing cards. As she finds each card, she must battle its magical personification and defeat it in order to seal it away.