A child named Marcus Preston has a birthday party where a costumed character entertainer of Chuck E. Cheese vomits and dies from having overdosed on painkillers as the police are unable to locate the source of the drugs.
At school, Marcus hosts an assembly mourning the recent deaths of multiple costumed characters due to overdoses from painkillers, including the now-deceased Swiper.
At the retirement home, the boys dress up and sing as a barbershop quartet while Stan finds McGillicudy's room and puts the Hummels into a large sack.
Club gave the episode an A− rating, and he noted in his review that "As a society, we worsen drug epidemics by keeping people relegated to the awful circumstances that drove them to sell or use the stuff in the first place.
I'm not sure what else Trey Parker and Matt Stone would have to say about the matter outside of the general shittiness of it all, and because 'Hummels & Heroin' works in service of story escalation rather than a sharpened moral, it ends up being the first truly gut-busting episode of season 21.