It satirises the questions young kids often ask about Santa by flipping the script and having Santa ask them, about a young boy named Harold. As much as Santa wants to believe in Harold, since he loves reading his letters, he's found some things about him, such as how the letters seem like they were written by his mother, Harold has changed appearance over the past year, etc.
Meanwhile, Harold is feeling the same way about Santa— he wants to believe in him, but finds the idea hard to believe. So, both try to see if the other exists.
This book provides examples of.
- Cassandra Truth: Santa initially thinks Mrs. Claus and Harold's parents are lying about Harold's existence, but actually they're telling the truth.
- Christmas Elves: We see Merpin, a short, pointy-eared elf in a green shirt and hat.
- Christmas Town: We get to see Santa's cottage and the "computer gizmo department" where Merpin the elf works.
- Con Man: Discussed, when Santa says that perhaps Harold's parents are lying that they have a son to trick him into giving them presents.
- The Cuckoolander Was Right: Downplayed. Santa's theory about Harold's non-existence was pretty wild and mostly incorrect, and he was wrong when he said that Harold was too young to bring out the snacks. However, he was right when he said that Harold's dad is the one who leaves the snacks (though Harold does help).
- Eskimos Aren't Real: Discussed when the rumour spreads and people start to think Santa doesn't believe in children.
- Faeries Don't Believe in Humans, Either: Downplayed— Santa is a human himself but he has magic, which Harold has his doubts about, and he doesn't disbelieve in all humans, just Harold. Even then, he used to believe in Harold, but stopped, hence the title. Jack Frost's parents and the Abominable Snow Monster also don't believe in Harold, though it's unknown if they believe in other humans.
- How Can Santa Deliver All Those Toys?: Harold's reason for doubting Santa's existence is wondering how he can do his job. He's seen reading books about Santa, drawing diagrams, and taking notes.
- Mall Santa: Santa himself apparently worked at a mall the previous year, where he was surprised that Harold had had a growth spurt that drastically changed his appearance.
- One-Steve Limit: Exaggerated— When Santa brings up Harold, with no last name, Mrs. Claus somehow knows which "Harold" out of all the boys in the world he's talking about.
- Reverse Psychology: Harold's mother tries to get him to come and open his presents by saying, "Too bad we don't know any little boys who want to get presents from Santa".
- Santa Claus: Santa is the main character, and the conflict of the story is him and Harold not believing in each other.
- Spit Take: When Harold's dad sees Santa, he spits out coffee.
- Unnamed Parent: Harold's parents are unnamed.
- Vague Age: Harold is old enough to use logic and read a book, but young enough to have lost a tooth the previous week. That's all we know about his age.
- Voodoo Shark: In-Universe— when Santa tries to explain Harold's appearance change as him not existing and his parents tricking Santa, Mrs. Claus is left confused as to why they'd do that. He thinks of two explanations— either they feel that if he finds out they're childless it'd ruin his Christmas, or they're conning him out of gifts.
- Weather Report Opening: The story begins with "one stormy night".
