In the opening scene, Homer tries to vote for Barack Obama in the presidential election, but the voting machine is rigged to turn his vote into one for John McCain, and after six attempts to vote for Obama, Homer then angrily rumbles the machine for not letting him vote Obama and it sucks him in and kills him. It then spits out his corpse and Jasper puts an 'I Voted' sticker on his head.
In the opening scene, Homer tries to vote for Democratic Senator Barack Obama in the 2008 American presidential election. However, the voting machine is rigged to turn his vote into one for Republican Senator John McCain. After six attempts to vote Homer heads out to report the mishap ("This machine is Rigged!" Must Tell President McCain!) but the machine sucks him in and kills him, then shoots out his body out of the voting booth. Jasper sticks a "I Voted" patriotic-themed sticker on Homer's forehead.
It is Christmas, and Bart has bought gifts for everyone except for Lisa. He enters a You Forgot-Me-Nots store in hopes of finding a cheap gift for Lisa. Upon entering the store Bart goes through a box entitled "Only 99 cents" and looks through the various toys and novelty items and subsequently gives up. Behind him a red cab-over semi-truck (reminiscent of the original Optimus Prime) appears, blinking its lights at Bart in an attempt to gain his attention. He dismisses the toy stating "Come back when you're something else!". The truck rolls back and transforms into a pink convertible Ferrari. The car beeps and calls Bart's attention. Bart turns around and exclaims "Perfect!". On Christmas morning Homer hands Grandpa a gift-wrapped tank of Oxygen as a gift. Grandpa responds like an eager child to "play with it outside" and after confirmation runs to the front yard to breathe in the Oxygen, but collapses on the lawn. Bart gives the pink convertible to Lisa. She then sarcastically remarks the last Christmas gift she received from Bart, a box of burps. Bart then grabs her Christmas stocking and proceeds to burp in it. Lisa opens the box and to her surprise gets "an actual present". She then gratefully hugs Bart. Meanwhile the convertible transforms and emits a laser beam to burn the angel on top of the Christmas tree, Maggie drops her pacifier as it turns back into a car. Maggie points at the toy to alert Lisa, but she doesn't buy it. The toy then grabs the pacifier and shoves it into Maggie's mouth. That night the toy transforms and "chokes" the Malibu Stacy doll. The transformer then shoots 4 beams at an alarm clock, a lamp, a boom-box and another off-screen. The convertible transformer calls the "Posibots" to transform: Alarm Clock "Snooze", Lamp "Three-Way", Boom-Box "Melody". The Posibot Leader notices a missing member, "Sex Toy". When he enters the room, the pink ferrari transformer asks him where he was, to which he responds "Where haven't I been!" The next morning all the machines in the kitchen have been converted into transformers and transform while Homer reads the newspaper. He then notices something "different" to which the transformers unanimously state "no". Even the toaster transforms to "NO"! Outside Homer enters his vehicle and backs out of the driveway, but then the car transforms with his head sticking out the back (resembling Optimus Prime in color and design). Another robot comes (which resembles the original Megatron) with Ned Flanders in its posterior and both robots proceed to fight. Homer then asks Lisa what is happening to which she responds with a surprisingly correct theory. All over Springfield machines transform into robots. In the center of town "Carnage Destructicus" & "Bestimus Mucho" prepare for a final battle. Marge interrupts asking for the fighting to end. The Transformers realize they have no reason to fight in the first place and thus team up to enslave Humanity. Members of the Springfield community are then seen tied to poles as players of a Foosball game which Homer suggested the Transformers might enjoy after Moe demands who is responsible who taught them.
Homer and Marge put Maggie into a daycare center, but Maggie is upset by them leaving. And so, Homer turns her attention to a mural with Krusty's face on it, since she's familiar with the clown. However, the real Krusty the Clown defaces sections of the mural with his face on it, as it's an unlicensed use of his image. Homer then shoves Krusty in retaliation because he made his daughter very upset. Angered, Homer pushes Krusty into a swing which swung him into a merry-go-round that spun him onto a pony, then a trampoline into a wood chipper, killing Krusty. Later on, Homer is approached by businessmen who heard of his deed. Apparently, the likeness of dead celebrities can be put in commercials for free, since they refuse to sponsor certain products. They manage to convince Homer to start killing celebrities such as George Clooney, Prince, and Neil Armstrong. Meanwhile, back in Heaven, the dead celebrities (including John Lennon) are outraged by these exploits. Krusty the Clown convinces the dead celebrities except for Abraham Lincoln (who for some reason likes his exploit about him and George Washington kissing each other at a wedding commercial for a marathon, although this outrages Washington) to descend from Heaven and stage an attack upon everyone who benefited from these exploits. The dead celebrities start killing people at the party and shoot Chief Wiggum. Krusty is just about to kill Homer when asked "What is the one true religion?" to which Krusty replies "It's a mix of Voodoo and Methodist" and blows Homer's head off with a shotgun, killing Homer. When they return to Heaven, they find that Homer had locked them outside for revenge, and he became very, very good friends with Abraham Lincoln.
In a parody of It's the Great Pumpkin, Charlie Brown, Milhouse is waiting in the pumpkin patch for the Grand Pumpkin on Halloween, and Lisa accompanies him because (as she puts it) his glasses fog up when he cries, which he would probably do to learn that the Grand Pumpkin doesn't exist. Soon Lisa leaves in frustration when she sees everyone at school having a Halloween party By Using his childlike belief, Milhouse's tears bring the Grand Pumpkin to life. However, the Grand Pumpkin is appalled to find that his kindred pumpkins are being carved up on Halloween, and vows revenge by eating several people whole, including Homer, Groundskeeper Willie, Nelson and kills Principle Skinner. Realizing that Milhouse can bring things to life by believing in them, Lisa tells him about "Tom Turkey", a symbol of Thanksgiving, who comes to life and kills the Grand Pumpkin, freeing from everyone he ate. However, when Tom Turkey learns of what people do to turkeys on Thanksgiving, he vows revenge and begins eating some of the children as well. Where The scene then cuts to Marge who says to the audience that they can send complaints to the following address and wishes everyone Happy Holidays.
In "It's the Grand Pumpkin, Milhouse", the characters were redesigned to resemble the style of Peanuts, and they also obtained rights to use Vince Guaraldi's music. Also, Bart uses Charlie Brown's catchphrase "Good grief" and is wearing his clothes. "Untitled Robot Parody" is modelled on the live action film, rather than the cartoon. Al Jean said it was "just really fun to do transformations [and] you can see why they enjoyed doing that film." "How to Get Ahead in Dead-Vertising" featured a parody of the title sequence of Mad Men. Jean was a fan of the show.
Rick Bentley, of the Seattle Times, described "Treehouse of Horror XIX" as a "paint-by-numbers episode." Robert Canning of IGN gave the episode a 7.9/10, calling it "funny, entertaining and even nostalgic [which] only makes this yearly tradition that much better."
"It's the Grand Pumpkin, Milhouse" was regarded by reviewers as the best segment in the episode. Canning wrote, "this segment may not be all that gory, but it's funny and, quite honestly, it will just make you feel good." Bentley described it as "a dead-on comedy assault of the Charlie Brown animated Halloween special." Rob Owen concurred, writing that it, "succeeds because it offers sly cultural commentary." Showpatrol wrote "The nostalgia factor makes “Grand Pumpkin” the best of these amusing bits for me, but they all lack that trademark “Simpsons” brand of satirical smartness."