Squidward walks outside his house on a sunny day only to find out that his yard is covered in Gary's slime trail. He attempts to keep Gary away by building a splintery wooden fence around him, but to no avail. As he complains to SpongeBob about his pet, Gary bites him. Squidward asks SpongeBob if Gary has had all of his shots, and SpongeBob says that he has, until Patrick comes along and asks him if Gary has been vaccinated for "Mad Snail Disease," a disease that turns anyone bitten by the afflicted into a zombie, which SpongeBob says he has not.
Suddenly, Gary runs off and starts to bite everybody in sight. Bikini Bottom is sent into a state of mass hysteria, and destruction, and soon, almost everybody has been bitten and turned into zombies. Those who have survived the disease are hiding out at the Krusty Krab. SpongeBob tries to get in, but Patrick believes he is a zombie and will not let him in until he slips through the doors. Old Man Jenkins suggests that they might all be zombies, and everyone starts panicking. Gary then comes in, much to the horror of the onlookers, and SpongeBob tries to bring him to his senses, but he gets bitten, much to his horror. The survivors in the Krusty Krab then decide to terminate Gary.
However, a doctor named Gill Gilliam enters the building to put an end to the commotion, explaining that the disease is just an urban legend and that the supposed symptoms were all in their imaginations, and that Gary was simply suffering from "Grouchy Snailitis" due to a "tiny" splinter in his foot from Squidward's fence, which he removes. SpongeBob is happy that Gary was never disease-ridden and asks if he still loves him with Gary meowing in response. The remaining people outside appear to hear this, and even though they are not infected, Squidward still acts like a zombie due to the fact that he is now working at his job and starts serving customers. The episode ends with Gary biting Mr. Krabs for no reason at all.