Seven-year-old orphan Emma wishes for a puppy on Christmas Eve, but after misspelling “Santa,” her letter is accidentally sent to Satan, Lord of Hell. Curious about the strange human wish, Satan sends Shadow, a creature from Hell, to Earth. Instead of fearing him, Emma gives Shadow warmth and love, and the hellhound gradually stops hurting others.
Intrigued, Satan comes to see Emma himself, only to be mistaken for Santa Claus. To protect her, he hides his identity under the name Stan and stays by her side. From repairing her broken home and learning to cook to celebrating her birthday and creating Christmas miracles, the ruler who once believed fear was the only true power slowly learns what family means.
But Satan’s use of magic attracts Heaven’s attention. The angel Gabriella is sent to investigate and discovers that the legendary demon is actually protecting a child. Even after learning Stan’s true identity, Emma still chooses to trust him.
When an ancient demon imprisoned by Satan for a thousand years awakens, Emma uses a feather left by Gabriella to summon her. Angel and demon join forces to defeat the enemy.
In the end, Gabriella takes human form and stays on Earth, claiming she is “on vacation.” By the next Christmas, a demon, an angel, and a little girl have become an impossible but loving family.
