"The Simpsons Already Did It" is the seventh episode of Season Six, and the 86th overall episode of South Park. It aired on June 26, 2002..
The episode was inspired by the fact that The Simpsons did in fact beat the creators of South Park to an idea. In the episode "The Wacky Molestation Adventure" from Season Four, Cartman was supposed to block out the sun. However, one writer revealed that "The Simpsons already did it." In response to this episode, the creators of The Simpsons added a scene in "The Bart of War", where Bart and Milhouse watch an episode of South Park and ironically criticize the fact that adults are voicing the child characters.
This episode debuts an alternate version of the opening sequence for Season Six. It is mostly the same as the one used up until the episode "Professor Chaos," except for inclusion of clips from that episode, clips of Tweek from "Gnomes" and "Tweek vs. Craig" (the replaced clips were of Ms. Choksondik, Butters, and Butters's parents), and the inclusion of Tweek in the crowd shot. Butters, as Professor Chaos, comes in and knocks down the "South Park" sign at the end.
Professor Chaos is frustrated when he realizes that all of his diabolical plans to wreak havoc on South Park have already been done by "The Simpsons". Meanwhile, Cartman creates a kingdom of sea people.
Cartman excitedly shows Kyle, Stan, and Tweek an advertisement he found for "Sea People" (a parody of Sea-Monkeys). Cartman (led along by images from the advert) imagines them to be a race of fish-form people similar to mermaids who will "take me away from this crappy god damn planet full of hippies." He convinces everyone, including a very skeptical Kyle, to chip in so they can buy them.
That night, Cartman dreams (and sings) about going to live among the Sea People, so that by the time morning comes he is very worked up about it and thus quite disappointed when the Sea People turn out to be nothing more than brine shrimp. In order to get at least some amusement from their purchase, the boys decide to put them in Ms. Choksondik's coffee. She then dies (as had been announced in the previous episode two months before).
Upon hearing on TV that semen had been discovered in the teacher's stomach, the boys come to the conclusion that they inadvertently killed Ms. Choksondik with their "sea men." That night, they go to the morgue to steal the sea men evidence, fearful that they'll "find the women too!" Eventually, Chef explains to them that there is a difference between "sea men/semen" and "Sea People", and that the brine shrimp couldn't have killed their teacher. By this time, the semen they recovered had been added to the Sea People remaining in Cartman's aquarium. To his extreme surprise, when Cartman wakes up the next morning he discovers that the semen have caused the Sea People to rapidly evolve into a thriving society of about Sumerian or Babylonian technological advancement, which leads him to propose his own Theory of Composite Dynamics: "Sea-People + Sea-men = Sea-Ciety."
Meanwhile, Butters, in his evil alter-ego persona Professor Chaos, is still trying to figure out a way to bring disarray to the town. When he plots to block out the sun, his assistant, Dougie/General Disarray, informs him that it mirrors a plot of Mr. Burns' from The Simpsons. Frustrated, Butters abandons the idea and instead decides to cut the head off of the town's central statue — which mirrors Bart Simpson's decapitation of Springfield's Jebediah Springfield statue. On the news report, the newscaster interprets Butters' vandalism as homage to The Simpsons and even reports that the police are not going to investigate the crime because they want the statue to remain headless as a tribute to The Simpsons. Butters continues to devise other, increasingly outlandish, evil schemes, but Dougie keeps pointing out that every one of Butters' plans have also already been done on The Simpsons call them Homer Simpson, Marge Simpson, Bart Simpson, Lisa Simpson and Mr. Burns.
In an attempt to come up with an original evil plot, Butters watches every single episode before introducing his newest evil plan: build a machine that replaces the cherry centers of chocolate covered cherries with rancid mayonnaise (a plot that Dougie/General Disarray dismisses as being too unoriginal to appear on The Simpsons). Right as Butters is about to put his device into use, a Simpsons commercial announces that Bart is going to do exactly the same thing in that night's episode. Butters has a nervous breakdown and begins to have hallucinations and picture the town in the animation style of The Simpsons.
Back at the Cartman household, the boys have bought more Sea People, a larger aquarium, and several gallons of semen. (Cartman says, "Yeah, I got most of the sea men from a bank, but the rest was from some guy in an alley named Ralph. He didn't even charge me, I just had to close my eyes and suck it out of a hose!") Their Sea-Ciety continues to evolve, into an Ancient Greek-esque civilization, and they begin to worship Cartman as a god, even building a statue. Much to Cartman's delight, accepting his divine kingship gladly by shouting, 'I am god of the Sea People! I am master of their great sunken empire!'
Stan and Kyle invite Butters, along with others, to come see the aquarium. Hysterical, Butters realizes that the Sea-Ciety plot is very similar to that of the Treehouse of Horror VII short "The Genesis Tub", wherein Lisa Simpson brings to life a civilization in a tooth. The boys don't see much problem with this, noting that in its long run The Simpsons has done everything, so worrying about that is pointless. Chef then points out that they in turn borrowed their ideas from elsewhere. Butters understands and snaps out of his hallucinations, and everyone quickly returns to their normal appearance. At that point, Butters happily leaves, getting ready to wreak havoc once again, in which everyone proudly compliment him to do so. Meanwhile the Sea People on the other side of the aquarium begin to worship Tweek and build a statue of him, leading to a holy war, complete with suicide bombings. Only minutes later they develop nuclear weapons and completely destroy themselves and the tank.