Fourth grader Sakura Kinomoto finds a strange book in her father's library -- a book made by the wizard Clow to store dangerous spirits sealed within a set of magical cards. But when Sakura opens it up, there is nothing left inside but Kero-chan, the book's cute little guardian beast... who informs Sakura that since the Clow cards seem to have escaped while he was asleep, it's now her job to capture them!