We open with kids standing in line waiting to meet the mall Santa (Patton Oswalt). A bearded Kumar (Kal Penn) arrives and cuts right to the front where he sits on Santa's lap, telling him his Christmas wishes. The Santa indulges him for a bit, and then says to meet him after work. Kumar later meets the mall Santa in the parking garage where he shows Kumar his collection of Christmas-themed chronic (i.e. It's a Weediful Life). Kumar buys some weed and returns to his apartment.
Meanwhile, we find Harold (John Cho) in a suit at a huge office tower, presumably working in the financial market. His assistant comes in with a huge box. The box turns out to be a present for Harold's father in law - a huge 56 inch aquos plasma TV in... 3D! Harold is pleased and says he is going to meet his "friend" downstairs for a ride home. As Harold and his assistant leave the office, they attract the notice of some angry protestors, similar to the Occupy Wall Street group. The assistant is egged mercilessly as Harold flees to a waiting minivan - not driven by Kumar but by Todd (Reno 911's Tom Lennon). The protestors egg, urinate, and then throw shit at Todd's car as he makes a very slow getaway through NYC traffic. We learn that Todd hopes to be Harold's best friend, and he drops Harold off at his plush suburban home. Harold's house is decked out in fancy Christmas decorations, and Harold explains that his father-in-law is obsessed with Christmas and hopes that the decorations will impress him.
We cut back to Kumar as he is smoking in his dingy and dirty apartment. He finds a memento and chucks it in a box labeled "Vanessa" (Danneel Ackles), suggesting that the two have broken up. As Kumar prepares to smoke some of his freshly-purchased weed, his geeky, glasses-wearing neighbor Adrian (Amir Blumenfeld) drops in. He says that he has been invited to a party that night at a girl's house whom he met over the internet. Adrian is excited because the girl says she is a virgin and wants to give herself to him.
We cut back to Harold's house. As he walks in, his smoking hot wife Maria (Paula Garces) is waiting. She quickly undoes her robe to reveal her sexy red lingerie. She tells Harold that she is ovulating, and they begin having sex on the stairwell. A knock is heard, and they quickly end their dalliance as Maria runs upstairs. Harold answers the door to find Mr. Perez (Danny Trejo), Maria's father, glaring at him. Perez is clearly unimpressed with Harold's work. As he steps aside, we see that Perez has unexpectedly brought along his whole large family as well, including one relative who has clearly just escaped from jail (and threatens Harold with a sinister hand gesture).
We cut back to Kumar's apartment as he sits there enjoying his weed. A commercial comes on TV introducing the WaffleBot, a walking/talking robot. The WaffleBot is the hot new Christmas gadget. It bakes fresh waffles and douses them in warm syrup ready to serve, so kids don't have to stick forks in microwaves or pour scalding syrup on their hands. Kumar is clearly impressed, but surprised a few seconds later when Vanessa walks in. She is clearly disgusted by the mess in the apartment and Kumar's beard. Eventually, Vanessa reveals that she is pregnant, and the baby is his (since she hasn't had sex with anyone since they broke up 3 months ago). Kumar is too high to respond coherently, and Vanessa walks out on him complaining that he never can finish anything, including medical school (Kumar reveals that he failed to get his medical license because he failed a drug test). As Kumar sits down to resume his bong, he hears a knock at the door. When he answers, nobody is there, but someone has left a package addressed to Harold, who hasn't lived there in years.
Switching back to Harold's place, Perez sees that Harold has erected a snow-white artificial tree in his house. Perez is incensed and says they need a real tree. When Harold asks why, Perez tells his life story. He grew up extremely poor (and had a mustache at the age of five) when he immigrated to America. His mother promised that every year they would have a Christmas tree. On their first Christmas, while Perez's mom is walking home after work with a bag full of ornaments for the tree, she is attacked and killed by a Korean gang. That, he says, is why trees are so important. Luckily, he has brought along a 12 foot tall Douglas Fir tree that he grew himself. Maria and Harold discuss the Christmas party they will be hosting next evening, and though Maria urges him to invite Kumar, Harold refuses noting that it is natural for friends to drift apart. Perez notes that the tree cannot be naked and says he will skip Midnight Mass to decorate it. Harold, hoping to impress him, says that he will stay home instead while Maria, Perez, and family attend Midnight Mass downtown. The family leaves as Harold prepares to decorate.
We cut back to Kumar, who has shaved and is leaving with Adrian for the party. On the way, they plan to drop off the package at Harold's house. They pull up to Harold's house, and both are shocked at its size and decadence - clearly Kumar has never been there. Kumar goes to leave the package on Harold's steps, but slips on some ice on his way back to the car, alerting Harold to his presence. Harold invites Kumar in for a drink and talks about his successes. Harold opens the package, and it turns out to be a GIANT blunt. Neither of them know how or why this was sent to Harold. Kumar lights it up and Harold grabs it from him, saying he doesn't smoke anymore and throws it out the window. A gust of wind blows the blunt back through another window and it lands on the Christmas Tree, lighting it ablaze. The two unsuccessfully try to put it out, eventually tossing the burnt tree out the new bay window Harold had recently installed. Harold and Kumar argue, and Kumar leaves with Adrian.
Todd comes over with his young daughter (about five) Caren (Isabella Gielniak) to see Kumar gift-wrapping his window shut. Caren keeps crying, and Todd says that he can't find any songs that calm her down. Todd eventually tells Harold that he has located the last 12 foot Douglas Fir tree in NYC and has reserved it online. They go off together to pick it up. Meanwhile, Kumar and Adrian arrive at a Christmas tree center to pick up a new tree for Harold, with Kumar texting Vanessa that he is finally seeing something through. The owners of the center are two black men, one very aggressive and threatening, the other lovable and subservient. It turns out (although Kumar doesn't know it) that they are the ones holding on to Harold's reserved tree. He sees the reserved tree and buys it for three-times its sale price. They tie it to the top of Adrian's car and go off to Harold's house. As they begin to leave, Todd and Harold pull up. The black men have changed clothes, teeth and wigs so that they can try out each other's stereotypical roles. They tell Harold they sold his reserved tree, and point to Adrien's car as it speeds away. Todd and Harold follow in hot pursuit, and eventually Adrian ends up flipping the car, blowing it up, and destroying the tree.
Harold and Kumar fight again, as each insults the other's new friend. Adrian is upset that he can't make it to his party to meet the internet virgin, Mary (like the Virgin Mary) (Jordan Hinson). He shows Harold an internet video of Mary, and Harold sees that Mary has a 12 foot Douglas Fir Christmas tree. After receiving false assurances from Adrian that they can have Mary's tree, Harold and Todd agree to take Adrian and Kumar to the party. Kumar runs into Harold's car and hotboxes it with a joint, unaware that Caren is in the car. Caren gets a contact high and gets the munchies. As Caren starts to freak out, Kumar sings a very vulgar Wu Tang Clan song to her, which ends up soothing her. Todd is impressed that the song worked, and even more impressed that Harold also knew the words to the "rap-hop" song.
They arrive at Mary's opulent house, full of drunken and drug-using teens. Mary and Adrian immediately go to her bedroom, where Adrian begins to question how a beautiful girl like her had never gotten laid. Meanwhile, Mary's younger brother refuses to let Harold and Kumar take the tree unless they beat him in a game of beer pong. Todd, meanwhile, keeps looking for empty rooms to change Caren's diaper in, but can't find any. In one room, he meets a bunch of teens sniffing cocaine, and when one of them sneezes, the cocaine gets all over Caren causing her to begin rubbing her teeth and gums. Meanwhile, as Adrian and Mary are making out, he sees a picture of Mary's dad and realizes her dad is a violent and notorious Ukrainian mafia don. He freaks out and loses his hard-on. Seething with anger that this has happened again (because of her dad's reputation, nobody will have sex with her) Mary goes out to the party to find someone else. The first person she sees is Harold, who she forcibly begins trying to give oral to. Kumar tries to pull her off when her dad returns home with two of his henchmen. It appears that Harold and Kumar are the ones assaulting his daughter, and he orders his men to kill everyone at the party. The teens, Harold, and Kumar successfully flee the apartment. The father sends his two goons to find Harold and Kumar and kill them. Todd, Adrian, and Caren fail to escape and are stuck hiding in a closet right in the mafia don's presence. Caren, being high as a kite, begins crawling on the ceiling.
Harold and Kumar discover that the drinks the teens gave them were drugged. Because of that, they begin to hallucinate that they are Claymation figures. They are then chased by a homicidal dirty giant snowman. As the snowman is about to kill the both of them, they clear up and discover that they were attacking plastic snowmen. They find that they were followed by Rosenberg and Goldstein (the Jewish drug dealers from the first two films) (Eddie Kaye Thomas and David Krumholtz) and Goldstein's son who talk them down from their high. They all decide to go to White Castle, which is just across the street from where Kumar and Harold were hallucinating. While eating, it is revealed that Goldstein has converted to Christianity, and claims to be much happier (his son's name is Christian). It turns out that all of them and Rosenberg's uncle were invited to Harold's party - a fact that annoys Kumar. The duo decide to leave White Castle to head back out. Goldstein is upset that Harold and Kumar shortchanged him $0.83 for the bill, to which Rosenberg responds that he hasn't lost all of his Jewish roots.
As they leave, they discover a Church with a Douglas Fir in it. Kumar concocts a plan where he will incapacitate the head priest, sneak into the nun's shower room (involving graphic nudity), have Goldstein's son dress as an altar boy to distract the rest of the priests, have Harold cut the power, and steal the tree. As they consider putting the plan into action, they see that this is the Church where Maria, Perez and family are attending Midnight Mass. Perez seemingly catches sight of Harold and goes up to confront him, only to discover it is another Asian man (Perez notes that they all look the same). Harold and Kumar had ducked into an alley to avoid Perez. Kumar says Harold shouldn't have to go through all this trouble for Maria, and Harold agrees, saying he doesn't HAVE to do this, but he WANTS to in order to make Maria happy. As they stand in the alley, a person opens a door and pushes them both inside.
The duo is soon dressed as nutcrackers, and before they know it, are thrust onstage to perform with Neil Patrick Harris (referred to as NPH) as part of his holiday musical show. Harold sees that the stage is decked out with the exact type of Christmas Tree he needs. After a brief musical number, the two follow Neil to his dressing room. On his way there, Neil discusses how he is so busy doing charitable work and acting on How I Met Your Mother, and kisses his boyfriend in front of the press. Harold and Kumar sneak into the dressing room and discover that NPH is not really gay, and that his boyfriend is actually his drug dealer. A hot, female backup dancer comes into the dressing room, and NPH convinces her to undress for him, so he can give her a massage. As NPH begins to masturbate while massaging her, the girl realizes he isn't gay and leaves the room in a furor. NPH then discovers Harold and Kumar hiding there. NPH says he only acts gay to pick up chicks, and that Clay Aiken uses the same trick. They ask how he managed to stay alive since they saw him get shot to death at a whorehouse in Texas in the last movie. He reveals that he went up to Heaven after being shot and gained access to the nude nightclub there. Unfortunately, Jesus caught him getting a hand job from both of Jesus' girlfriends, and convinced god to send Neil Patrick Harris back to Earth. They also discover that NPH has a WaffleBot, which he abuses verbally and physically. Kumar says he loves the WaffleBot and fucking hates pancakes. NPH agrees to give the two the tree they need and WaffleBot, and says he looks forward to seeing them again in the fourth film.
As Harold, Kumar, and WaffleBot walk down the street, the two begin to argue again over their failed friendship. As they are arguing, Harold gets a call from Vanessa, and Kumar discovers that even she has been invited to Harold's Christmas party. The argument escalates, when the two mafia goons ambush and drug them and WaffleBot. When Harold and Kumar awaken, they are tied up in their underwear to a metal pole. They are doused with gasoline, when the henchmen begin to argue because they realize that they forgot to buy a lighter. Harold tells them to stop arguing, and urges them to realize that even if they are in different places in life, they shouldn't throw away a lifelong friendship, even if one of the friends is a screwup. Kumar responds, saying that the least successful goon shouldn't be jealous of the more successful one, and maybe the lesser one should try to be more mature and less jealous. The goons hug and make up, but still insist on killing Harold and Kumar. At that moment, WaffleBot recovers and takes out the two henchmen, scalding one in the face with boiling waffle batter and hitting the other in the crotch and face with the waffle iron. WaffleBot then uses its warm syrup shooters to melt the bonds tying Harold and Kumar. As its battery dies, WaffleBot states that it loves Kumar and it also fucking hates pancakes. Their troubles are not over yet when it turns out that Harold's penis is frozen stuck to the metal pole (similar to the kid's tongue in A Christmas Story). Kumar eventually frees Harold by accidentally dropping a full cup of scalding hot coffee on his privates.
The two go outside, realizing that it is too late to salvage the night as Maria and family are surely home from Midnight Mass. They are in the middle of nowhere, and their cell-phones are not working. Harold reveals that he only quit smoking so he wouldn't become impotent, but hasn't been able to have a kid despite trying for a year. Kumar then reveals that he is going to be a father, but totally blew it with Vanessa. Searching for a way home, Harold reasons that people always come running to the sound of gunshots, so they shoot one of the henchmen's guns into the air to attract attention. Instead, the real Santa Claus falls from the sky with a headwound. Santa begs to die, but Kumar uses his medical knowledge to patch up the wound and save his life. Santa flies the two back to Harold's house in his sleigh, and notes that Harold is a good boy, and that is why he gave Kumar the giant blunt so that they could rekindle their friendship. Harold invites Kumar to his party the next night, and Kumar gladly accepts it. Santa flies off with Kumar.
Meanwhile, still stuck in the closet, Caren begins freaking out and saying she needs more drugs. The mafia don hears the commotion in the closet and pulls out his gun. When he opens the door, Caren runs out at super-speed, and to the tune of Matrix-style music, challenges the don to a fight. Todd sneaks up behind the don and tries to knock him out, but fails. Before he is shot though, Caren manages to knock the don out with a heavy metal object. Caren then begins eating the ecstasy that she found in the don's pockets. Todd, believing they are mints, takes a few ecstasy too, and he leaves with Adrian and his daughter.
As Harold enters his house, he finds Perez waiting for him in the dark, demanding to know what happened with the tree. Harold recounts his story and ends by saying that Perez needs to show him respect so long as he is in Harold's house. Perez is impressed at Harold's display, and says that he is happy to know that his daughter married a man that is nice, but also has cojones when needed.
Kumar is dropped off at Vanessa's parents' house and sneaks into her room. He says that he has learned a lot and is ready to grow up. He says that he doesn't HAVE to be a dad, but WANTS to be a dad, and that he loves Vanessa. Kumar even offers to get his medical license and give up pot. Vanessa accepts him back, but tells him not to give up pot, but just to ask her for her urine when he takes his drug tests. The two then have sex.
The next morning, Harold awakens to two miracles. Maria is pregnant, and a Douglas Fir Tree has been delivered. Santa's bloody Christmas Cap lies near the hearth. The next night, everyone arrives at the party and has a good time, and Perez watches the movie we have just seen on his new 3D TV (while wearing the same 3D glasses as the audience). Todd is sweating and rubbing his face all over, while Caren has acquired glowsticks and a neon pacifier and is raving in her carriage. Harold and Kumar share a smoke outside their house, finally reunited. As the day begins to break, we see Santa's sleigh returning to the North Pole, with Santa smoking pot from a candy-cane joint.
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