The very first episode of Trey Parker and Matt Stone's television series, South Park, debuted on August 13, 1997 (this series followed two short films by Parker and Stone, both titled "The Spirit of Christmas," that they made in 1992 and 1995. The latter one was made into a Christmas video card, of sorts, that was passed around so much that it got the attention of Comedy Central, leading to Parker and Stone getting a chance to make a pilot for the network, which led to them getting their own series). This first episode was titled "Cartman Gets an Anal Probe" (Parker and Stone were definitely trying to push the boundaries of good taste right off the bat) and it told the story of how a group of alien "visitors" were visiting the small town of South Park, Colorado and performing anal probes on the citizens, including young foul-mouthed fourth grader Eric Cartman. The visitors abduct the younger brother of one of Cartman's classmates, Kyle Broflovski, and Kyle, Cartman and their two other friends and classmates, Stan and Kenny, must rescue Kyle's brother, Ike. Eventually, the boys use the anal probe that the visitors left in Cartman's ass to lure them back to Earth in time to rescue Ike.
The visitors then leave...or DO they?
You see, while the aliens left, they did not leave for good, as they began appearing in the backgrounds of episodes of South Park, leading to the very popular claim that aliens have appeared hidden in every episode of South Park since. A quick Google check will bring you many references to this fact, like here from OMG Facts. Have they really appeared in every episode of South Park since, though?
No, no they have not.
Essentially, what we have here is something very similar to what happens with the urban legend about every episode of Seinfeld containing a reference to Superman (something I addressed in a Comic Book Legends Revealed years ago). People can't seem to be happy with something cool like this happening occasionally. Instead, it has to be exageratted to be happening all the time.
The alien sightings were never in every episodes, but they did appear with a great deal more frequency in the early seasons, including in the opening credits. As time went by, it seems as though the animators found the joke not as interesting, as the appearances became a good deal more sporadic.
They picked up again in 2008's Season 12, though, as Comedy Central had a contest where fans could win stuff by finding the hidden visitors in each of the first seven episodes of Season 12.
South Park Studios have addressed this legend a few times over the years in the Frequently Asked Questions section on their website. In 2002...
How many episodes have "hidden visitors" in them?
Over half of them. Eric Stough, our Director of Animation, usually puts them in at the last minute. That is... if he remembers to. Unfortunately, he doesn't keep track of them, so we don't have exact numbers.
and in 2010...
I'm sure you've answered this a thousand times but I cannot find the answer: Is it true there is an alien in every episode?
In EVERY episode?? No way. But the Visitors are there quite a bit -- sometimes right in plain sight, sometimes with something as hidden as a little Visitor doodle.
The legend is...
Check out this list of a BUNCH of examples of hidden Visitors on cracktwo.com.
Feel free (heck, I implore you!) to write in with your suggestions for future installments! My e-mail address is email@example.com.