Things at the water park are not what they seem. The boys's fun filled day is about to turn deadly. Events are in play at Splash Town that signal the end of the world is upon us. Cartman tries to warn everyone but no one will listen. Disaster is about to strike and the boys are in a race to survive.
Cartman, Stan, Kyle, Kenny, Jimmy and Butters visit Pi Pi's Splashtown, where Cartman is horrified to discover the entire park is filled with African-Americans and Latin Americans, who he referred to as "minorities" (despite there being 60% of minorities to 40% of "normal people"), and just after viewing the "minorities" he sings his ballad "Minorities at my Water Park". At the same time, Kyle discovers to his disgust that almost everyone but himself urinates in the pool. He then leaves and refuses to re-enter. Cartman tries to warn the other boys that through calculations, he predicts that by 2012, the world will be devoid of white people and the other races will take over. His friends then simply dismiss him for being racist and leave.
Meanwhile, a scientist urges the owner Mr. Pi-Pi to shut down the water park, explaining the water in the park is 98% urine and 2% water, where the safe levels are below 83% urine and if he doesn't shut it down, the park will fall victim to a huge event. Inevitably, Pi-Pi's assumed lawyer refuses to shut the park down. It doesn't take long for the pool to get to 100% urine, and when it does, a large tidal wave of urine sweeps across the park, destroying all in its path. Cartman manages to stay afloat in the pee, Kyle climbs up a high water slide, and Stan, Butters, and Jimmy get to higher ground atop the Mount Everest ride. Unsurprisingly, Kenny doesn't make it.
Meanwhile, the scientist explains that anyone with prolonged exposure to the urine will 'mutate' leaving the victim angry and violent, demonstrating this by urinating on a helpless monkey, and naturally the monkey gets angry. The military order a vaccine to be formed, without considering that the monkey is just a bit annoyed at being pissed on. After Kyle has joined Stan, Butters and Jimmy atop the Mount Everest ride, Cartman has been rescued from the urine by a group of survivors aboard a Floom Zoom raft. Shocked to realize he is the only "non-minority" onboard, he believes that the Mayan prophecy has occurred 3 years early; he is quick to imagine a world in which he must speak in minority slang, is paid lower wages, and eventually forced to live in a concentration camp. The other boys meet up with Mr. Pi-Pi who has also survived (and is clearly incredibly disappointed with himself). He tells them of a release lever in one of the basements to drain all of the urine, and says that anyone who goes down there would need to stay down there for an extended period of time. Naturally, Kyle is chosen (due to him claiming to be able to hold his breath the longest).
The military produce a vaccine, but it fails, resulting in three dead monkeys. Kyle is forced to drink three cups of urine, so he can safely travel down into the depths without his body having to endure the effects of fluid pressure. A now incredibly irritated Kyle insists that he will not, despite that if he doesn't, they will all die. Another vaccine is formed, which turns out to be a simple banana (once again demonstrating the military's stupidity), they test the cure out on Stan's father who, while very annoyed that they were peeing on him, doesn't mutate as everyone believed he would. As Kyle finishes his third cup, a helicopter flies in and saves them, much to his anger having drank pee for nothing.
Outside the park, Cartman is reunited with the others, where he realizes that what they escaped from was not the Mayan 2012 Prophecy, and they still have three years left. The boys still don't understand Kyle's anger at drinking the urine. He finally snaps, and tells them that he is grossed out by pee, and that the only thing he hates more than pee is bananas, which the military then hand out and force the boys at gunpoint to eat to stop the 'mutation', much to Kyle's outrage.