Stan, Kyle, Cartman and Kenny decide to play detective; they go around the town solving minor "crimes" for a dollar.
After bringing Butters in for "questioning" and ordering him produce a semen sample, they get a tip-off that Bill and Fosse have the doll.
They manage to recover the doll from them, despite the interference of another group of kids playing FBI who attempt to assert jurisdictional privilege and take over the case.
The criminals open fire at the "police," but end up killing themselves, destroying the lab, and causing a great deal of property damage to the surrounding area.
The boys are next sent to a strip club owned by Gino, the leader of Colorado's largest drug cartel, which operated the original meth lab.
As they play in Cartman's basement, Butters emerges from the bathroom and proudly announces that he was finally able to produce a semen sample by imagining Stan's mom's breasts.
On the DVD commentary, Trey Parker said that the episode is based on his childhood, where he had a desk and a rolodex in his parents' basement and played junior detective.
[1] "Lil' Crime Stoppers", along with the fourteen other episodes from The Complete Seventh Season, were released on a three-disc DVD set in the United States on March 21, 2006.