Some info and a short description of this South Park episode on South Park Zone:
South Park - Good Times with Weapons is episode 112 of South Park and the first episode of Season 8. It originally aired on March 17, 2004. The episode's animation routinely switches from the usual cutout-and-solid-color style to a highly stylized anime theme.
In this South Park episode, the boys meet a vendor selling "authentic weapons from the Far East." They buy Japanese weapons and they pretend to be ninjas, and they transform into anime-like characters having their individual superpowers, but when they hit Butters with a throwing star, they must rush to get Butters medical attention without getting caught.
South Park Spoiler Alert!
(The complete plot for this South Park Episode)
At the South Park County Fair, our boys come across a vendor dealing "authentic weapons from the Far East." After tricking him into selling them several dangerous ninja weapons (parental permission is required if the hopeful buyer is under 18; Cartman uses this information to pretend that he, Kenny, Kyle and Stan are orphaned brothers, a ruse the boys have apparently successfully used "like twelve times" in the past), they pretend to be ninjas, only to transform into anime-like characters with their own individual superpowers, as the animation style takes on an overall Japanese look and switches to a Cinemascope aspect ratio. The boys refuse to let Butters play, so he becomes his alter ego, Professor Chaos, and sets off to fight them in a stylized anime sequence. The sequence ends abruptly when Kenny throws a shuriken at Butters, whom they all see as Chaos, which hits him in the eye and gets stuck. To avoid having to explain themselves and getting into trouble with their parents by taking him to a regular hospital along with playing with weapons, the boys decide to dress Butters up as a dog and begin their journey to the local veterinarian.The boys hide Butters in an abandoned stove before facing off with Craig's gang, who have also acquired ninja weapons out of envy. After an epic battle, the boys realize that Butters has disappeared and try to find him before he can tell anyone, enlisting Craig's gang in the search. Meanwhile, Butters makes his way to the hospital, but his disguise fools the doctor, who sends him to the animal shelter. At the shelter, the still-wounded Butters is urinated and defecated on by multiple other dogs. The elderly animal doctor, also fooled, makes no attempt to treat him, preparing instead to put him to sleep. Before he can do so, however, Butters escapes, so the doctor simply says, "Oh, well. Let's murder one of these other dogs."
The boys decide to dispose of the evidence and return to the fair to have the vendor refund their weapons, but Craig and the others inform them that they have seen Butters wandering around on the other side of the fair auction. Cartman pretends to be invisible to get to Butters undetected, unwittingly exposing himself onstage to the shocked audience. Butters stumbles onto the stage a few moments later and collapses. The final scene shows the townsfolk protesting at an emergency meeting about the outrage at the auction. The boys are under the impression that the outrage in question is Butters' wound, but it soon transpires that the real issue is Cartman's public nudity. Cartman explains that it was a "wardrobe malfunction," and the episode ends with the other three boys addressing the issue that adults are more offended by s3x over violence, allowing them to keep their weapons.