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On a snowy Christmas Eve, Tyltyl and Mytyl sit in their home. Normally, they would be caught up in the Christmas spirit, but their mother has taken ill. So that evening, they lie awake, unable to sleep, when through the window crashes a strange, mercurial magical spirit named Berylune, who reveals that the world we live in isn't the only one there is. And so Tyltyl and Mytyl, accompanied by various spirits, as well as their now-humanoid pets, are led on a journey to find the blue bird of happiness and save their mother, and along the way they visit some strange and wondrous places, and finally, must face the spirit of darkness to attain their elusive goal.

(Source : themanime.org)