Garfield goes trick-or-treating as Catzilla for Halloween, while the real Catzilla escapes from the zoo.
On Halloween night, Garfield reluctantly answers the door for trick-or-treaters. With one trio that arrives, he gives each of them candy. He also eats the rest of the candy from the bowl, prompting Jon to buy some more. When more trick-or-treaters arrive, he gives them nothing and closes the door. When Jon tells him to give them something unneeded, he tries to give away Odie. Jon finally gets some taffy left over from last year, with Garfield assuring him that he won’t eat them.
Garfield wonders what he should dress up as for trick-or-treating. He figures that the scarier his costume is, the more candy he’ll earn. Turning on the TV, he learns about Catzilla, a saber-toothed tiger who is considered the scariest creature around. Garfield then makes a costume to look like Catzilla (using black paint for stripes and a toy walrus for tusks). He then tries to make himself hostile to fit the personality. He imagines scenarios with Vito, a gym instructor, and Nermal, the latter of which does the trick.
Meanwhile, Jon turns on the TV for the news, where he and other people find out that Catzilla has escaped from the zoo. As Garfield goes trick-or-treating, people become terrified by Garfield, who they think is the real Catzilla. While Garfield does well with getting treats (except for peanut brittle), reports are made of Catzilla around the street.
Eventually the police arrive to capture Catzilla, forcing Garfield to go on the run. When he arrives at his house, he tries to get Jon to let him in, but he and Odie also think he is Catzilla. Jon reports the sighting to the police; they soon catch Garfield and bring him to the zoo. However, no one except Garfield sees the real Catzilla sneaking into the house.
At the zoo, Garfield tries in vain to escape, and then realizes he can take off part of his disguise to convince a zookeeper that he is not Catzilla. Upon being freed, Garfield rushes back home to warn Jon and Odie. He arrives shortly after Jon and Odie encounter Catzilla, which results in a chase sequence around the house. Eventually, Garfield manages to tame him by giving him taffy, with the police finally catching him.
The next day, Jon, Garfield, and Odie see Catzilla is safely back at the zoo. When feeding time comes, the zookeeper informs them of Catzilla’s diet, which includes various foods (such as lasagna), and peanut brittle for dessert. Delighted, Garfield decides to join Catzilla for dinner.