Penn doesn't want to celebrate Christmas because his parents won't be there. However, he is sent to the North Pole to fill in for Santa Claus to save Christmas.
Penn Zero as a knight (along with his friends Boone as a wizard and Sashi as a fairy) reaches the end of his quest to find a sword in a stone. He pulls on the hilt and a spatula emerges. He moves to the next rock, which houses a toilet brush, and the next which is a bouquet of flowers. The flowers are singed by a breath of fire sent by the dragon Rippen is riding. Rippen knows that the sword is truly in a couch as opposed to a stone, so he snatches it and flies to the safety of his castle.
Penn then rides on a catapult which has been enchanted by wizard Boone. Rippen commands his minions to shoot flaming arrows at the heroes, but they all miss. Fairy Sashi, sitting in the launcher of the catapult, picks one up as they speed by. Penn snatches a battle ax from one of the opposing goblin soldiers and uses it to cut the rope and launch Sashi at the castle. She falls a little short and Larry quips that she is an "Air ball". However, unknowingly to them, she throws a vine around a part of the castle and break the wooden poles supporting the tower on which Rippen stands. He plunges into the moat, and Penn surfs across the water to reclaim the sword. He slides across the bricks, knocks down some goblins, avoids a cauldron of lava, and lands in the throne room to put the sword back on its stand. It transforms the castle back to its true form as a colorful castle. The gang fist bumps and is returned back to their own realm. Boone says that he misses his face hair.
Penn wakes up on the day before Christmas, believing there is snow falling outside, but when he opens the window, he sees Sashi on his roof beating the stuffing out of a reindeer decoration. Penn calls his parents on the MUHU, but he sees their message recording of them fighting flying sharks. He sadly leaves a message saying that he wishes they could be there for Christmas, but he is understanding that they are trapped in the Most Dangerous World Imaginable. Before he can finish the message, he loses connection.
When Penn goes downstairs, his Aunt Rose and Uncle Chuck proudly show off their "Egg-O-Rama", a sculpture of everything Christmas-y made out of pigeon eggs. The Chinchilla tries to eat some but is disgusted by it. Boone, on the other hand, enjoys the eggs. Uncle Chuck then begins an egg ball fight.
To cheer Penn up about his parents not being there Boone presents him with skateboard sleds. Sashi also tries to cheer up Penn by removing everything that might make him feel sad about his parents being absent on Christmas. When they arrive at the abandoned Odyssey theater, they see Rippen and Larry walking into the building next door. Penn asks Phyllis for a favor to help repair his MUHU, but she is unable to do that. Penn hopes that this mission will keep his mind off of Christmas until the team is sent to the North Pole.
Boone (as a reindeer) sits on Santa Penn's lap and tells him everything that he wants for Christmas. Sashi as an elf tries to encourage him, but Penn accidentally knocks his pals down with his new round belly. If the gang does not recover the elves and launch the slay before a blizzard comes at midnight to cancel Christmas.
They infiltrate Santa's workshop to discover that the toys are destroyed. They find a box which contains the head of a snowman inside. He asks for them to help him, telling them that the rogue gingerbread army caused all the damage. They spy on the toy factory, where the gingerbread men (led by Rippen and his henchman Larry) are building weapons to destroy Christmas. The team arms themselves with Christmas-themed weapons and head to a secret entrance that the snowman knows.
As soon as Penn, Sashi, Boone, and the snowman get past the cookies standing guard, Rippen spots them and has his minions attack them. During the scuffle, Penn drops his MUHU. The snowman thinks he sees it down a hallway, but at the end is a trap set by the snowman himself. Leaving the heroes in a cage, the evil snowman reveals that he is working alongside Rippen. The villain declares that Christmas is canceled.
Though Penn is distraught sitting in a cage, his friends do their best to make him feel better. Boone and Sashi sing saying that they'll break out before Christmas. As they do so, magical objects attempt to break them out, but it does not work.
Rippen has a feast for his minions because he believes they have succeeded. In the back room, Penn recalls that his parents told him that Santa can slide down any chimney. He attempts to simulate a chimney using a roll of wrapping paper, but he is unable. Boone suggests that maybe his friend has to go "full Santa": he has to put a present under the tree. With a sack of presents, Penn barely squeezes out.
The trio crash the party. Boone devours the gingerbread cookie soldiers, while Penn leads a brigade of nutcrackers. Penn spots the trapped elves, and begins to roll over the bad guys, buy Larry stops him by aiming a frosting blaster at him. Penn realizes, however, that Larry cannot pull the trigger because he ate his own gingerbread fingers. When the elves are released, Penn rushes to the sleigh in order to save Christmas.
Snowman tries to stop them, but Penn rearranges his snow and turns him into a snowsnail. Boone pulls the sleigh up towards the launch platform, but along the way, Rippen jumps on and begins to fight Santa Penn. Using the toys in the magic sack, the hero and villain fight. When the MUHU starts ringing, Penn must decide whether to take the communicator back or save Christmas. He chooses the honorable decision and causes Rippen and the MUHU to be thrown off into the snow far below, where they both shatter.
After the reindeer are successfully harnessed, the sleigh takes flight and flies past the blizzard in the North Pole. With the victory of the mission, the team is zapped back to their home base. That night, Penn falls asleep in his living room. He awakens on Christmas morning, but does not see any presents under the tree. Just out of sight, however, is a small box making a vibrating sound. Attached is a note that says "To Penn, You dropped something... Santa" and inside the box is the repaired MUHU, which is receiving a call from his parents who wish him a Merry Christmas.
- Henry Winker as The Snowmen