Plot
A young Mariah Carey has wanted a dog her entire life, but isn't allowed to have one due to her father's allergy and partly due to her mother's cleanliness. A few days before Christmas, her grandma takes her to the pet store and she sees a hypoallergenic poodle, who she names Princess and immediately falls in love with. She tries to convince her dad to let her have the dog for Christmas, to which he agrees, but only if she proves she's responsible enough to own one.
He brings home a scruffy Jack Russell terrier named Jack, who he says is owned by his brother (and Mariah's uncle) Reggie. Jack proves to be more problematic than anticipated: He runs out of the house refuses to come back in, but Mariah manages to lure him back inside with a steak. While she picks up his poop, Jack gets frightened by the gear she's wearing and pees in the hallway. When she tries to take him on a walk, he breaks loose of his leash, runs through the town park, and knocks over Mariah's principal. Mariah tries to convince her she's not his owner, but her principal scolds her for lying. At her choir concert, Jack barks along with the music so loudly he causes some snow on the pavilion to fall onto the choir members. When she tries to deliver some toys too a donation box, Jack refuses to let them go due to having grown an attachment to them. Mariah tries to yank him out and accidentally knocks over some Christmas decorations.
While preparing for a dog show, Jack rips her Santa suit, causing her to have an emotional breakdown. Her sister helps her come up with a plan to borrow the suit off the Mrs. Claus animatronic from the yard of their neighbors, the Ingersols. Everything goes smoothly at the dog show until Jack vomits in front of everyone.
Mariah's grandfather Bill, annoyed that he can't put up lots of decorations like the Ingersols, constantly sabotages their display. One night, while trying to replace one of the blinking bulbs with a non-blinking one, he falls off the roof and gets tangled up in the lights. Mariah and her brother sneak over and help him land safely on a reindeer decoration whilst just barely avoiding detection.
On the night before Christmas Eve, two squirrels crawl in through the chimney and start playing with the decorations Mariah's family has set up. Jack chases them around, unintentionally creating a mess. The family comes downstairs to see the commotion, and Mariah angrily reprimands Jack for all his misbehavior. Feeling dejected, Jack runs under the couch and later out of the house. Mariah's grandma reminds her that Jack's only a puppy, and Mariah tries to apologize to him before going back to bed.
The next morning, Mariah is worried about how Reggie will react now that Jack is gone. Her father then reveals that Jack was actually just a stray Reggie found and he gave him to her to see if she would change her mind about having a dog. After everyone voices their disapproval, he apologizes and offers to take her to go get Princess. At the pet store, another kid is having fun with Princess but is saddened when he finds out that Mariah had put tabs on her. Feeling sorry for the kid, she lets him have Princess and realizes just how much she cares for Jack. Her and her family organize a search party. Mariah decides to look in the donation box and finds Jack with the toys she donated.
During the night of Christmas Day, the family chases the pair of squirrels around the yard, and it's revealed that her father is actually allergic to the squirrels rather then dogs. Bill, finally having been allowed to decorate the outside of the house to his hearts content, has made a very bright display, which ends up temporarily blinding Mr. Ingersol. While all this is happening, Jack helps himself to the family's roast.
