It is Halloween night and Garfield the fat orange cat has already eaten all of the candy which was intended for trick-or-treaters, leaving nothing but some stale taffy which was left over from the previous year's Halloween. In order to get some more sugary treats, Garfield decides to go trick-or-treating himself. He has difficulty choosing a costume but decides that he has to look scary, reasoning that the scarier the costume is, the more candy the trick-or-treater receives. Turning on the TV in search of inspiration, he sees a news report about a new arrival at the zoo, Catzilla, a rare descendant of the prehistoric saber-toothed tiger and the scariest animal to be seen at the zoo for a long time. Garfield decides that Catzilla would make the perfect costume for him. He paints black stripes on his front, to match those on his back, takes some tusks from a toy walrus to complete his disguise and practices looking ferocious in front of the mirror.
As Garfield leaves the house, his owner Jon and the dog Odie hear another news report. It says that Catzilla has just escaped from the zoo and that residents are warned to be on the look out for the dangerous creature. Garfield is unaware of the news.
At each house that he goes to, Garfield is mistaken for the real Catzilla. All the people who see him are so scared that they drop their entire bowl of candy in fright. The bowl lands in Garfield's bag each time. The cat is happy to have received twenty-three bowls of candy at twenty-three houses but is disappointed that his haul does not include any peanut brittle. Garfield's fun comes to an abrupt end when the police arrive, believing him to be Catzilla and wanting to take him back to the zoo. Garfield runs home but when his owner Jon sees him outside, he calls the police to tell them that Catzilla is outside his house. As Garfield is taken away to the zoo, the real Catzilla sneaks into Jon's house.
At the zoo, Garfield is placed in Catzilla's cage. He tries to escape without any success, before he realizes that he has only to remove the toy walrus tusks to show that he is not Catzilla. When a zoo keeper sees that the animal in the cage is just an overweight house cat, he has no choice but to let Garfield go.
Garfield arrives home just as Catzilla is about to attack Jon and Odie. Knowing that giving him something to eat would keep Catzilla occupied for a while, Garfield feeds him last year's stale taffy. Catzilla has great difficulty chewing the old candy and the sticky taffy glues his teeth together, making him temporarily incapable of biting anybody. Soon afterwards, the police arrive and Catzilla is returned to the zoo.
At the end of the episode, Jon and Garfield are visiting the zoo. The zoo keeper who had freed Garfield earlier tells them that it is Catzilla's feeding time and that the animal is about to be fed large quantities of steak, salmon, roast beef, lasagna and peanut brittle. On hearing this, Garfield decides to enter Catzilla's cage and join him for his meal.