Christmas episode where Canada and America are at war, and then the Norks get involved, and then Russia and China get involved, and it turns into World War 3. Then Hell opens up and Satan invades Earth again, but then the forces of Heaven arrive to do battle with Satan. But then eldritch horrors are released by Crab People and the visitor aliens arrive to enforce cancellation upon whole planet on behalf of the intergalactic FCC. Then the Super Best Friends along with uncensored Muhammad show up to stop the aliens from cancelling the Earth, and the final battle takes place in South Park. Things just keep escalating and getting more fucked up. An episode in the style of 200/201 with the plot of Bigger Longer Uncut put into overdrive. A true South Park armageddon.
Meanwhile main characters are dying left and right Mars Attacks style. Randy dies, Garrison dies, most of the fourth graders die, Stan dies, even Cartman dies. Heidi goes full Tetsuo with evolving waterbear powers to stop the aliens and the Great Old Ones and finally Mr. Hankey sacrifices himself to reset continuity all the way back to 3rd Grade. Next season Garrison is teacher again, South Park is a small isolated mountain town, and the kids are back to being naive 8 year olds.
But the reset didn't eliminate all continuity. It only fused the timelines, so a lot of newer characters like PCP and Jimmy are still present and much of the continuity from later seasons is still roughly in effect.