Garfield comes into Jon's bedroom and requests a breakfast, and when kindly asking does not work, he and Odie demand "pancakes the size of Australia," prompting Jon to wonder "if people with goldfish have this problem." After his pancake breakfast, he prepares to kick Odie off the table and notices a trip to the vet that Jon has scheduled on his calendar. In an attempt to make Jon forget the vet appointment he marked on the calendar, Garfield forcefully shoves the page into Odie's mouth. He then notices that tomorrow is Thanksgiving and becomes excited. Jon and Garfield then first head to the grocery store to get the foods and ingredients for Thanksgiving. After Jon finishes the grocery shopping, Garfield quickly notices that they are not heading home, and Jon reveals that they are heading to the vet. Garfield screams in panic and starts throwing the food at Jon, causing him to hit the curb repeatedly.
At the vet's office, Garfield continues to panic and insists that Jon scheduled this appointment to "check out the vet," rather than give him an actual checkup. After Liz recommends that Jon put Garfield on a diet, she grudgingly agrees to go out with Jon to stop him from holding his breath until she does so when she refuses several times. When Jon takes Garfield home, the instructions provided by Liz recommends that Garfield only gets half a lettuce leaf, which makes Garfield lethargic and more on edge. To ensure that Garfield cannot cheat, Jon has Odie keep an eye on him. When Garfield reluctantly finishes his leaf of lettuce, he checks his weight on RX-2, his talking scale. RX-2 insults Garfield in numerous ways. Garfield stomps RX-2 and hurries away. He attempts to cheat by sneaking a cookie to ease the unhappiness. Odie pops out of the cookie jar and loudly blows a whistle in his face every time Garfield attempts to steal sweets from the cookie jar. This attempt at cheating quickly escalates into a cat and mouse game, where Garfield tries to outsmart Odie and get his hands on a cookie, leaving Garfield furious and exhausted.
The next morning, Garfield is in a foul mood and Jon, not wanting his happiness spoiled, tries to prepare the Thanksgiving dinner. During the process, he skips several essential steps, including, dethawing the turkey 24 hours in advance. Removing the giblets and filling the cavity with stuffing, not buttering the turkey, and cooking the turkey at 500 degrees instead of the cookbook's recommended temperature. Garfield then sprinkles garlic powder all over the other Thanksgiving ingredients to ensure that everyone else cannot enjoy the holiday. Jon attempts several fashion choices before Garfield suggests that he dress as himself. While Jon checks on the turkey and Thanksgiving ingredients, Liz thinks that the diet might be a little hard on Garfield. She thus allows him to skip the dietary requirements, not wanting him to suffer during Thanksgiving. Garfield, realizing his awful mistake, rushes to the kitchen where he suggests to Jon that he call Grandma. Grandma fixes everything and undoes Jon's many mistakes while he distracts Liz. She first takes Jon's partially thawed and cooked turkey and turns it into turkey croquettes. Grandma then makes sweet potatoes, cranberry sauce, and pumpkin pie using a homemade recipe. With everything all fixed up, she quietly leaves and returns home. The Thanksgiving dinner turns out wonderfully, and at the end, everyone agrees that they have Grandma to thank. When Jon suggests he, Garfield, and Odie take a walk to work off the dinner, Garfield gets up from the sofa, but not Odie, causing Jon to put him on a diet and Garfield appointed as his trainer.
The book adaptation, which retains the original title "Garfield's Thanksgiving," deviates in the following ways:
- When Odie and Garfield wake up Jon, they demand pancakes the size of China, instead of Australia.
- Jon only holds his breath until Liz accepts his invitation for his Thanksgiving dinner, not a date.
- While being examined, Garfield spends the whole session silently in denial, before going into shock from being sentenced to a diet, and then passing out at the same time as Jon holding his breath.
- Odie and the scale are not involved in Garfield's torment, and Jon does not notice Garfield's depression as he is too happy about having Liz over for Thanksgiving dinner. Garfield is also weak and suffers from insomnia.
- Garfield's sabotage of the Thanksgiving feast is considerably nastier because rather than garlic powder, he squirts detergent and cleaning products all over the Thanksgiving ingredients.
- When Jon shows Garfield the ruined dinner, he shows a ruined pie and sabotaged vegetables; the food is not destroyed in the special.
- Jon is the one who decides to call Grandma.
- In the end, there is no mention of any diets.
- Garfield's Thanksgiving is the last television special with music and lyrics by Ed Bogas and Desirée Goyette.
- Garfield's Thanksgiving is the last television special animated solely at Film Roman.
- Four out of the five people in the vet's waiting room reappear in Garfield's Diner as customers.
- The talking scale, RX-2, irritates Garfield by calling him Orson Welles and claims to be a fan of Citizen Kane. When Garfield destroys the bathroom scale, its last word is "Rosebud," the very first line from Charles Foster Kane.
- One of Jon's outfits is Popeye.