Set right where the last episode of season 19 of South Park has left off, the boys have survived a chemical plant explosion while trying to reduce the blast radius and stop a cloud of toxic fog from engulfing their nearby city of South Park, CO, attempt by Nathan, Mimsy, and their villain friends excluding the reformed PC Principal (voiced by Chuck Norris). They have then been back in town after the sole but presumed demise of Eric Cartman whose been replaced by Katara aka Clayface as Butters has begun experience mass amounts of bad luck when the other boys during lunch at the South Park Elementary cafeteria 1 month since the events of S19 have warned him of salt knocked over resulting in bad luck.
Soon, they find out this is the following work of an evil snail (voiced by Christopher Lloyd) who is behind Butters' bad luck from inside his noggin. As the boys (and Katara whose impersonating Cartman, as Clayface) try to rid Butters of the big bad bug, the real Eric Cartman has turned out to have also survived the chemical plant explosion outside South Park and become a bit feral.
South Park S20 premieres Sept. 21st at 10pm on Comedy Central.