Aunt Flo comes to stay, but her goldfish she bought for Stan Marsh turns out to be evil. Also "Cartman" grows a beard and starts acting nice!
At the bus stop after school, Stan, Kyle and Kenny are discussing the pumpkin carving contest and how they should enter. They realize that Cartman is not at school again and wonder what is going on. Cartman then shows up, but he has a beard. He tells the others that he couldn't come to school since he was looking after his mom because she was ill. He asks if any homework was assigned. The boys are shocked and confused and tell him about the pumpkin carving contest and Cartman offers to get money so they can buy a pumpkin.
Later on, Cartman shows up but without the beard. The boys ask him where the money is, and he responds angrily with open hostility and excessive use of the word "hella".
At Stan's house, the boys begin to carve a squash Kenny bought into a jack o'lantern when Cartman comes in but with his beard again. He shows Kenny how to cut in a way he won't hurt himself, then breaks into song.
In the night, Stan puts a shirt over the goldfish bowl to hide the threatening messages appearing on it. The fish then seems to move closer and closer and Stan screams, waking his family. He finds a dead body on the floor and screams again. Sharon comes in and buries the body in the garden believing it was Stan who did it.
After many more deaths, Officer Barbrady asks if Sharon Marsh knows anything about it. After he looks around she takes him captive in the basement for fear of him taking Stan away.
Cartman shows up at the bus stop and the boys tell him he can't be there - since the bearded Cartman is also there. Cartman gets wound up on wonders what has happened.
The boys then try to take the goldfish back to the pet store after it kills Kenny and Aunt Flo, where they find a gateway to a parallel world where the evil pets and bearded Cartman have come from in the pet store.
They leave the fish and go to pumpkin carving contest where they win. Suddenly a bearded Stan and Kyle show up and have released all the pets. Cartman has explained to them about his annoying counterpart and they have gone looking for him. Normal Stan and Kyle get the transportation to the parallel world device off their counterparts and send them back. They then decide to send Normal Cartman back since he's horrible. He then rips the beard off of the other Cartman and the boys are confused. One of the Cartmans says they'll both have to be destroyed, realizing he is the nicer Cartman they send Normal Cartman back as he shouts "Screw you guys!".
Good Cartman asks how they knew it was him, they say that the normal Cartman would never say that, he then reveals himself to be the real Cartman and tauntingly sings to the boys that they're "hella stupid".
Randy frees Barbrady who begins to try and stop the pets, to which Sharon realizes it was Stan's goldfish doing the killings.