This episode takes place two million years in the future, in which a robotic snowman named Mr. Sludgy is telling children the story of the "most horrible Christmas ever". It all started when Zim, GIR and Minimoose were on the streets trying to make monies; when the alien notices that fat men in red coats and white beards were making a lot of money. He captures one and he discovers the story of Santa Claus, in which Zim is amazed over how much power Santa has over the humans.
Zim decides to create a giant Santa suit that he can use in order to pretend to be Santa, and have more power over the humans and become their ruler. Dib figures out Zim's scheme and heads out to stop him in order to not only save Christmas, but to save the entire human race as well. Unfortunately, Zim has the support of every human on Earth, except for Professor Membrane, who has a grudge against Santa. Zim gathers the human race together and commands them to build a transporter for "Santa". Dib confronts Zim in front of everybody, trying to expose him as a fraud. Instead, the humans treat Dib as usually and Zim has him dragged away to the "Jingle Jail". Dib escapes from his prison on Zim's ship and disguises himself as one of Zim's robot elves. Zim, not realizing that the elf is actually Dib, starts venting with him in the room. Zim explains that he is using the santa suit to trick humanity into building a teleporter that will take them to the Tallest for enslavement. But the suit Zim is wearing is starting to take over him because it actually thinks it's Santa. Zim then sees through Dib's disguise and launches him out of the ship and onto a sinking piece of ice.
Gaz comes to rescue her brother in Tak's Ship, but only to beat him up after he confessed that he decapitated her doll. Dib then contacts his dad, who gives his son permission to use all of his anti-Santa arsenal. Armed with a giant robot suit, Dib confronts Zim again at the North Pole, where the teleporter is complete. Unfortunately for both Zim and Dib, the Santa suit has taken full control over the alien, and Dib is forced to battle it. In the end, Dib's robot suit defeats it, Zim is freed from the suit, and the mutated santa monster is launched into space. But it's not a happy ending. Zim, dressed as the Easter Platypus, convinces the human race that Dib killed Santa and they attack him.
With the story ended, we go back to Mr. Sludgy talking to the kids, and he explains to them that Santa is in fact not dead, but is in space gathering strength to attack the Earth every Christmas, and that is why they live in a protective shield.
- This was the only Invader Zim episode produced after Nickelodeon cancelled the series in January 2002, as Nick granted the Zim crew permission to make at least one more 22 minute length episode.
- Invader Zim: Volume 3 - Horrible Holiday Cheer
- Invader Zim: Season 2
- Invader Zim Wiki: The Most Horrible X-Mas Ever
- Christmas Specials Wiki: The Most Horrible X-Mas Ever
- "The Most Horrible X-Mas Ever" at TV.com
- "The Most Horrible X-Mas Ever" at the Big Cartoon DataBase
- "The Most Horrible X-Mas Ever" at the Internet Movie Database