Open: A 2 minute animation of Santa arriving at the Griswold family house and putting presents under the tree.
We open with Clark and Ellen singing Christmas carols in the car on the way to a Christmas tree lot out in the country. As Audrey and Rusty are increasingly uninterested in Clark's plan to put on the perfect Griswold family Christmas, Clark gets into a dangerous game of cat and mouse with some truck drivers after he's cut off and flips them off. This ultimately causes Clark to veer the car off the road and jump over 500 feet. As the Griswolds find their perfect tree, Rusty reminds Clark that he forgot to bring a saw.
Back home, as Clark gets out his Christmas decorations and begins setting up the tree, he makes an off color joke to his pretentious yuppie neighbor Todd about where he'd like to put the tree. That night, the excess sap causes Clark to stick to everything he touches.
The rest of the movie is told through the style of an advent calender. Starting with:
The Griswold family is at a busy department store buying Christmas presents. Clark can be seen hitting on "Department Store Mary" in front of his son Rusty. That night Ellen's parents and Clark's parents arrive at the house.
That night, Clark reveals to Rusty his plan of illuminating the house with some 25,000 imported Italian twinkle lights. Audrey tells Ellen that her plan of sleeping in the same room as her brother is sick and twisted.
Clark sets about illuminating the house with some 25,000 imported Italian twinkle lights, and a few other decorations like a plastic Santa. His first attempt at setting the lights up ends in near disaster with several broken windows and Clark stapling the sleeve of his shirt to the roof. The yuppie neighbor Todd and his wife Margo are complaining about Clark destroying their stereo and carpet in the process.
Clark is at a potentially very important meeting with his boss Mr. Shirley. He believes this is the key to landing a bigger holiday bonus. He and his associate Bill discuss what they're planning to do with their bonuses, and Clark reveals that he's going to put in a swimming pool.
Clark is nearly finished setting up the 25,000 imported Italian twinkle lights. As he makes for his big presentation, he finds that there's something keeping the big display from powering on. It's later revealed that a switch in the garage is what is keeping the display from turning on.
Clark goes to hide some Christmas presents he bought in the attic and discovers some that he simply forgot about several years ago. Unfortunately he's locked in the attic and the family thinks he disappeared working on getting the light display up and running. They give up on tying to find Clark and head for the mall. Now trapped in the attic, Clark passes the time by watching some old family reels of Christmas past.
That night, Clark is desperate to get the lights working. As the Griswold house is illuminated, it causes a massive power surge that requires the city's auxilary power to be turned on. As the family stares in awe, the neighbors nearly destroy their house, and Clark is stunned by the sudden and completely unannounced arrival of Cousin Eddie and his family.
The Griswolds decide to have some weekend fun and go sledding. Clark reveals his big project that he's been working on with his chemical company: an experimental kitchen lubricant. He greases up his sled and sails down the mountain at an unusually high speed, electrifying everything in his path.
Clark still can't believe Eddie crashed the party, and is stunned that he is emptying a chemical toilet out into the sewage drain underneath his house - which does not directly drain to the ocean. Eddie discovers that the experimental lubricant that Clark had applied to the sled had nearly destroyed it. While he and Eddie go shopping, Eddie reveals to Clark that he's flat broke and the RV is actually his real house. Clark then offers to get Eddie some Christmas presents.
Back at work, Clark reveals to his associate Bill that it's his last day of the year and drops off his Christmas present to his boss Mr. Shirley, who doesn't seem too grateful about all the presents he's getting from his employees.
Clark is seen staring out the window at his future pool, fantasizing about Department Store Mary, who is appearing to give Clark a memorable daydream striptease. He's quickly interrupted by Eddie's daughter who tells him she believes Santa isn't coming. Clark reassures her that the opposite is going to happen. Eddie tells Clark that he's not leaving until the end of January.
It's finally Christmas Eve. The rest of Ellen's elder relatives arrive. The Griswolds finally get to reward the fruit of their efforts to put on the perfect Christmas Eve dinner with a 20 pound turkey with all the trimmings. Clark has Ellen's 89 year old Aunt Bethany say grace. But Bethany due to her age has a very limited memory and instead recites the United States pledge of allegiance. Unfortunately they left the turkey in the oven too long and cutting into it means that they have nothing but scrap meat. Eddie's Rottweiler Snots drinks the water from the Christmas tree causing it to dry out. While that's going on, post dinner - Uncle Louis lights up a cigar and incinerates the tree in the process. Clark, desperate to relcaim his tree, instead takes a chain saw to one in his yard. Aunt Bethany's cat - which she had wrapped up and intended to give to one of the Griswold relatives - can be seen chewing on a strain of lights on the Christmas tree and incinerates itself.
Post dinner, Aunt Bethany notices a squeaking noise made. It turns out the tree that Clark just brought in is the home of a live squirrel. Chaos ensues. Snots chases the squirrel around the house, destroying lots of objects in the process. A frustrated Clark informs the family that no one is walking out on Christmas despite the fact that it has become a total disaster. He then takes the chain saw to the staircase and fixes the null post. Finally having enough of Clark's madness, after Clark destroys another window in their house, yuppie neighbor Margo finally goes over to the house to take some action and punch Clark in the face. As she approaches Clark, she's greeted by the squirrel, followed by Snots. A battered Margo goes back home and punches Todd in the face instead.
As the family begins to clean up from the disaster, Clark gets an envelope. He reveals to the family his plans of putting in the swimming pool, and if there's even money left over, he'll fly the rest of the family out to dedicate the pool. During the clean up process, Eddie discovers the remains of Aunt Bethany's cat and jokes about it spending all of its' nine lives. The envelope contains what Clark presumes to be his Christmas bonus. Unfortunately it's a check for the Jelly Of The Month club. Clark goes berserk, telling his family that he would like his boss in front of him with a big ribbon tied to his head. Eddie disappears.
Later, the family is gathered around for the traditional reading of the classic story "Twas The Night Before Christmas". Clark's dad Clark Sr. officially passes the torch to Clark to read the story as it's his house and his Christmas. Mid story, Clark notices Eddie with a rather familiar looking man heading toward the front door. It's Clark's boss Mr. Shirley. Eddie had fulfilled Clark's wish of wanting his boss to tell exactly how he feels about the bonuses getting cut. This causes the boss to have a change of heart after seeing how it affects his employees first hand, but not before his wife and the Chicago SWAT team shows up to destroy the house further.
After all the dust is settled, the family and the Chicago SWAT team are all gathered outside, where Uncle Louis lights his cigar once again. The sewage that Eddie emptied from his chemical toilet in the RV had caused a radioactive glow. Uncle Louis drops the lighter and that creates an explosion that launches the plastic Santa Claus in the air. Witnessing the explosion, mistaking it for fireworks and thinking that it's July 4th instead of Dec. 24th, Aunt Bethany begins singing the US National Anthem. The family stops fighting and joins Aunt Bethany singing the National Anthem and the SWAT team join in. The movie ends with them finishing the National Anthem and begins singing Christmas carols and Clark looking up at the radioactive Santa flying through the sky.