Harold (John Cho) and Kumar (Kal Penn) haven't been friends since their last adventure in Guantanamo Bay. As the two has separated and have gotten new friends. Kumar still gets high and lives in the apartment., he used to share with Harold. Harold has become a sucessful businessman and is married to Maria (Paula Garces). Kumar finds out Vanessa (Daneel Harris) is pregnant and he doesn't take it to well. Kumar gets a package st his door, but it is for Harold. Before going to a party, Kumar brings it to Harold, who is trying to decorate the tree that Maria's father brought along. As It is weed and when Kumar lights it up, Harold throws it out the window which boomerangs back to into the house and into the Christmas tree which catches on fire. Harold and Kumar and both of they're new friends go to the party to get a tree but they end up getting shot at after the two are accused of trying to have sex with a Girl as her father gets his henchmen to get them. But the guys escape and start to hallucinate after drinking drugged eggnog which they were given to by a kid they beat in a game. As they see themselves and the city clay-animated as they later meet up with their old friend, Rubeberg and Goldstein. Meanwhile, Harold and Kumar's new friends are stuck in a closet with each other and one of their's high baby as the gangsters are still at the apartment where the party was being held. Harold and Kumar still go to look for a tree until they are taken into a place where they dress up as nutcrackers and they see Neil Patrick Harrus as they do a rehearsal for broadway. Harold and Kumar question how he is still alive after what in the previous film. But they ask him can they borrow of those trees and he let's them have one and the new Wafflebot. The two are then kidnapped by the two gangsters and taken somewhere and are tied around to a frozen pole, but the Wafflebot saves then by burning the gangsters with syrup and untie then by burning the rope, but Harold's penis is stuck to the pole, so Kumar spills hit coffee on it. The two in the closet escape by knocking the father out. Harold and Zkumar try to get help by shooting a bullet into the air, but Harold ends up shooting Santa Claus (Richard Riehle) in the face as Kumar stitches him and the jolly man gives them to ride. Harold learns he doesn't have to do what he wants to impress him or not. Kumar goes to Vanessa's and say he will grow up for the sake of their unborn child and he will get his job back as a doctor. Maria tells Harold she is pregnant as the next day, Kumar and everyone else goes to Harold's Christmas party as they watch the new 3D TV and Harold and Kumar go outside to smoke weed and they are once again best friends.
- John Cho as Harold Lee
- Kal Penn as Kumar Patel
- Neil Patrick Harris as Neil Patrick Harris (fictionalized version)
- Danneel Harris as Vanessa Fanning
- Paula Garces as Maria Perez-Lee
- Danny Trejo as Carlos Perez
- Thomas Lennon as Todd
- Patton Oswalt as Larry Juston
- Amir Blumenfield as Adrian
- Richard Riehle as Santa Claus
- Neil Partick Harris said "See you in the fourth one"
- This maybe the last "Harold & Kumar" film but Adult Swim is developing an animated series pilot featuring John Cho but not Kal Penn due to his political duties.
- Kal Penn's work in the White House was mentioned in this movie.
- This is the first holiday film in the franchise
- Seth Goldstein has officially left Judaism and became a Baptist Christian because he felt that it is more exciting and invigorating. It is unknown if Andy Rosenberg doesn't want to be friends with him anymore or still wants to stay with Goldstein to convince him to return to being Jewish.
- The guys go to eat at White Castle in one scene and so in leftover boxes in Kumar's apartment
- There is one scene where Harold, Kumar, and the city are clay-mated
- Kumar's father did not appear in this film. He was present in the previous two.
- Kumar's imaginary female humanoid marijuana bag did not appear in this film. She appeared in the previous two. This was also the first film where Kumar did not have a sex with marijuana daydream sequence.
- This is the first film to use mostly original music and be void of any licensed rap music. In the previous two films, dozens of licensed rap music has been played and several licensed songs other than rap were used to convey the emotions for the scenes.
- The music arrangers for the film probably could not find any other licensed rap music to borrow in the film, or had to refrain from using them to save the budget, or simply chose not to put any in deciding that none related to the scenes of the film that dealt more with Christmas and holiday spirit than marijuana, sex or explicit content.
- John Cho played the Star Trek character Hikaru Sulu in J.J. Abrams' films Star Trek and later on in Star Trek Into Darkness. Adrien at one point called Harold "Sulu" in the Christmas tree purchase scene.
- This is the final film for the series, making it the first and only 3D film for the series.