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Garfield (voiced by Lorenzo Music) has to go to the vet on the day before Thanksgiving. He tries to fool Jon (voiced by Thom Huge) by ripping out the calendar day but this does not work.
While at the vet, Jon tries to ask Liz to dinner the next day, but she turns him down. Jon decides to hold his breath while she details the diet that Garfield must go on. Both he and Garfield faint. Liz changes her mind about the date.
At home, Jon gets Odie to use a whistle to hassle Garfield into his diet. Garfield becomes a sourpuss, destroys his scale and gets his revenge on Jon by ruining the vegetables. Liz arrives and realizes the diet is too strict for Garfield. With clues, Garfield gets Jon to call Grandma, who saves the day. Jon gets a kiss on the cheek from Liz and Garfield goes back to tormenting Odie.
The book adaptation, which retains the original title "Garfield's Thanksgiving," deviates in the following ways:
- When Odie and Garfield awaken Jon, they demand pancakes the size of China. Afterward, as he finishes his breakfast, Garfield quips that it needs less "pan" and more "cake".
- Only Jon causes a commotion in the room - holding his breath until Liz accepts his invitation for Thanksgiving dinner - while Garfield spends the whole session silently going from denial into shock from being sentenced to a diet before passing out at the same time as a suffocating Jon.
- Odie and the scale are not involved in Garfield's torment, and Jon - too deliriously happy about having Liz over for Thanksgiving dinner - is ignorant to the fact that Garfield is in misery. Instead, Garfield comes across a mirror on his way back from his first minuscule meal, and quips that he's lost something already - his sense of humor. He is also weak and suffers from insomnia.
- While Garfield is cranky from a sleepless night and a single lettuce leaf for breakfast, he cheers up quickly after deliberately sabotaging Jon's dinner (specifically the vegetables), and is even cooperative by helping him pick a suit for the dinner (despite not telling him he forgot his pants).
- When Liz comes and waits for Jon to serve dinner, she takes the moment to inspect Garfield. After Garfield pretends to be suffering from every single symptom Liz casually mentions, she relents, takes him off the diet, and only prescribes for him light exercises instead.
- Jon ruins his own dinner by not preparing any of it properly. In the end, the turkey he did not thaw or butter is in terrible state, the sabotaged vegetables are smoking, and the pie he made is burnt.
- Jon is the one to decide to call Grandma. While Grandma prepares dinner - and Garfield stays to watch - Jon stalls for time by telling Liz about Thanksgiving history, which works so well it bores her to sleep until Grandma is done.
- At the end, there is no mention of any latter diets, and while Odie and Garfield sit comfortably with their full bellies as Jon sees Liz out the door, the three agree unanimously that they have one person to thank: Grandma.