The story's about a young Mariah (voiced by Breanna Yde), who has always wanted a dog and has asked for one pretty much every single Christmas for as long as she can remember, but has never gotten it. When two of Mariah's friends (who both have dogs) ask for her help with a charity fashion show that they're organizing, Mariah agrees to help them before they inform her that they inform her that they're "short one girl with a dog." Now Mariah doesn't just want a dog—she needs a dog.
Later on, Mariah sees a puppy named "Princess" at a pet store and decides that she wants to adopt her—however, since having a pet is pretty big responsibility, Mariah has to prove to her parents that she can handle taking care of a dog before actually adopting one. So to prove that, she agrees to dog-sit her Uncle Reggie's dog, Jack, who proves to be a scraggly little rascal—in fact, he's considered the worst dog in the county! Jack turns Mariah's family's perfect holiday preparations upside down in hilarious ways.
While the holiday isn't exactly the Christmas that Mariah wished for, it ends up being more than she ever could've wanted.
The movie features examples of:
- Mixed Ancestry: Just like the real Mariah Carey, Mariah in the Christmas Special is portrayed as the daughter of a black man and a white woman.
- Troublemaking New Pet: While Jack really isn't necessarily a "bad" dog, he's still pretty rambunctious and causes quite a bit of trouble for Mariah and her family when they first take him in.