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 they believe 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.
If one thinks very carefully, the cause of everything in this episode is because SpongeBob allows Gary onto Squidward's property and also because Squidward made the poles. He could have just used the shocking cage like he used in "Breath of Fresh Squidward."
Mad Snail Disease may be a reference to the real-life disease, mad cow disease. However, the so-called "Mad snail disease" has the symptoms of rabies.
During the news report with Perch Perkins, a fish states: "It's spreading through the air," this is impossible because they are underwater, although the fish might not know this.
When SpongeBob is running to the bus, the "Bus Stop" sign is there, but in the other scene, it disappears.
During the scene where SpongeBob says to Gary, "I knew you weren't disease-ridden!" Frank's purple shirt sleeve turned blue. In the next scene, it changes back to purple.
When SpongeBob arrived at the Krusty Krab, the doors were locked. Later on, when Squidward walks in acting like a zombie, the doors were unlocked (no one was seen unlocking the door, but it is possible Squidward had a spare set of Krusty Krab keys and unlocked it off screen).
Nat is seen as a zombie but later, he is later seen as a normal customer in the Krusty Krab.
Pearl Krabs was listed in the credits for this episode. However, Pearl is not seen or heard in this episode.