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 a horrified Cartman discovers that 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 that almost everyone but himself urinates in the pool. He is disgusted by this, then leaves and refuses to re-enter. Cartman tries to warn the other boys that through calculations, he predicts that the by the year 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 Cartman.
Meanwhile a scientist is urging the owner Mr.Pi-Pi to shut down the water park, explaining that the water in the park is 98 percent urine and 2 percent water, where the safe levels are below 83 percent urine and if he does not 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 percent 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 on the Mount Everest ride (Stan pointing out when Kenny floats by face-down in the pee).
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 peed on. The boys meet up with the owner (Pi-Pi) who has also survived (shown to be 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 (this also demonstrates how fairly stupid the military are), 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 the boys still don't understand Kyle's anger at drinking the urine. He says very clearly 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 him at gunpoint to eat to stop the 'mutation'. Kyle doesn't react well.