Mrs. Marsh's monthly visitor, Aunt Flo, gives Stan a goldfish for a present. Stan thinks it's spooky, but his mother makes him graciously accept it. The fish begins to kill people and his Mother believes it to be Stan, not the fish. Meanwhile, there is another Cartman walking around, who is an alternate version of his normal self walking around South Park. They soon come to realize that all the trouble is coming from the Indian Burial Ground pet store, and they set out to stop it.