Aliens land in the Rockies and kidnap Kyle's little brother; Cartman tries to build muscle for a prize-winning ceremony; and the boys unwittingly set up camp next to an active volcano.
Scott conspires with Saddam Hussein to have Terrance and Phillip killed; Dr Mephisto is shot just as he is about to name Cartman's father; and officer Barbrady learns to read.
Cartman joins a choir touring Costa Rica; an epidemic of spontaneous combustion strikes; and Chef is entranced by the girl of his dreams.
Cartman is sent to a juvenile detention centre; quintuplets move into the town; and the boys try to develop attention deficit disorder.
Apocalyptic events threaten to end the world; Big Gay Al is announced as the boys' new scout-troop leader; and Kenny, Cartman and Kyle are duped into joining a magician's cult.
A weight-loss spokesman causes pandemonium; the boys and their parents are excited by the possibility of a free skiing holiday; and the boys arrange for Butters to appear on a talk show.
The boys attend an anti-war demonstration as an excuse to skip school for the day; Timmy and Jimmy form their own gang; and the boys are given detention after art class.
The boys find a cache of martial arts weapons; Cartman dresses up as a Japanese robot; and Jimmy buys steroids to help his performance in the Special Olympics.
Mr Garrison goes under the knife to become a woman; a concert is arranged in celebration of hippie culture; and the boys spot their opportunity to break into showbusiness by becoming talent agents.
The town is excited when Chef returns after travelling the world; Stan persuades local residents to buy hybrid cars; and Cartman and Kyle debate the merits of 'Family Guy'.
Stan tries to understand Token's feelings on racism; Cartman is caught in a compromising position; and an infestation of head lice plagues the school.
Cartman contracts a virus; the boys meet Britney Spears; and the kids try out a new type of drug - cat urine.
Kenny takes his new girlfriend to a Jonas Brothers concert; Cartman becomes a vigilante; and the town struggles to cope with the economic downturn.
Kyle and Butters are accused of being sex addicts; Butters is hailed as a literary genius; and Randy deliberately gets testicular cancer.
Kyle helps to develop the latest must-have gadget; Jimmy insults the Germans at South Park Elementary's first comedy awards ceremony; and the Prince of Canada's bride is abducted during her wedding.
Clyde ignores advice and leaves the toilet seat up after use; Cartman launches a gemstones network show; and humanity begins to evolve at a rapid pace.
Cartman becomes enraged by the discovery that the NSA is spying on the American people; Stan catches his parents watching `murder porn'; and Cartman has nightmares depicting him as Brad Pitt in World War Z.
In Season 18, the boys create a company, everyone goes gluten-free, Randy starts to feel the pressure of harbouring a secret and Timmy's new business threatens taxi services in the town.
In Season 19, Cartman angers the new school principal, the townspeople want to build a wall to keep out Canadians, Kenny takes a job at Mr Kim's City Wok and the boys play ninja.