Drugs are flowing into South Park. A passionate young health advocate has traced the source of these illegal meds and is about to expose Stan Marsh.
The episode opens at new kid, Marcus Preston's birthday party, where Marcus and his party guests are shown having a good time. Marcus' parents have a man donning a Chuck E. Cheese costume come up to the stage to entertain him, and his party guest. The man, under the influence of painkillers, poorly performs for the children, who all except for Marcus were disappointed. However, only moments later the man violently vomits, collapses on stage, and dies, horrifying the children, especially Marcus. Chuck E. is carried away on a stretcher by the ambulance, as Marcus promises to avenge him. The scene switches to Stan at the Shady Acres retirement home, who delivers two hummels to Marvin after selling the pillows Marvin gave him. When he reacts negatively toward the hummels Stan brought him, Marvin confesses to Stan that another resident at the retirement home is, in his own words, "top bitch". He goes on to say that he and the other residents must deliver Ms. McGullicutty's pillows to people in exchange for Hummels, which she regularly expects, inspects and collects from them. The retirement home shows itself to be very similar to modern day prisons.
Stan waits on the corner of the street for an entertainer dressed as Swiper the Fox to give him more hummels in exchange for another one of Ms. McGullicutty's crochet pillows. Before Swiper arrives, Kyle, Cartman, and Kenny appear to ask to come with them to Kenny's house to set off fireworks. Stan turns down their offer. The boys are upset with Stan, and agitatedly they ask him why he continues to sell pillows for Marvin. Stan admits to feeling bad for him. Immediately after Swiper picks up the pillow, and gives him the hummels. Later, Marcus holds a presentation in the school cafeteria, and successfully riles everyone up over the recent deaths of the costumed character entertainers. During the presentation, the boys discover that the pillows Marvin has been getting Stan to sell for him have drugs in them, and drag him to the bathroom to get him to explain. Stan says that he was unaware of what was in the pillows, but the boys are still afraid of what everyone would do to them if they ever caught them now that they've seen Marcus' presentation. The boys begin to avoid Stan.
Stan visits Marvin again at the retirement home, where he tells Stan that if he doesn't keep selling pillows, Ms. McGullicutty will get him sent to the quiet room, and she later appears to intimidate them. The attention switches back to Marcus who visits the Park County Coroner. There, he discovers that Hummels were found in the rectal cavities of the deceased entertainers. At dinner, Stan tries to persuade Randy into letting grandpa Marvin live with them again, but to no avail, as Randy admits to being annoyed by him. Stan continues to help out his grandpa, ordering antique hummels for him. At school, Marcus catches Stan in a lie and begins to suspect him. Stan asks the boys to help him clear everything up. Cartman agrees to help, and devises a plan. Meanwhile, Randy and Sharon visit Marvin and are intimidated by Ms. McGuillicutty, who farts on both of them as she had Stan and Marvin. An African-American entertainer dressed as Peppa Pig collapses onto the ground at yet another birthday party. Marcus appears at the party and speaks to "her" as she slowly dies. She tells him that old people love hummels. Leading Marcus to the retirement home. The focus shifts back to the boys who were dressed as choir boys and visited Shady Acres to sing to the old people, providing a distraction for Stan, and allowing him to sneak into Ms. McGullicutty's room to steal all of her hummels, and bring them to Marvin.
Marcus appears right behind Stan and snaps a picture of him gathering all of the hummels from Ms. McGullicutty's room. Stan pleads for Marcus to not show the police the picture, saying there's a much bigger picture than the one he is seeing. Stan tells him that he knows he misses Chuck E. Cheese and that if he agrees to help him, he'd be putting an end to the deaths of these entertainers. Marcus bursts into tears and agrees to help Stan. Ms. McGullicutty steps out of the boys' performance and heads to her room to find all of her hummels gone. She confronts Marvin, who thanks to Stan had all of the hummels in a bag, which he subsequently uses to brutally mutilate Ms. McGullicutty and her followers, making him the new "head bitch" of the retirement home. Ms. McGullicutty is sent to the quiet room to play solitaire, and the episode ends with Marcus confronting a group of health professionals, getting ready to interrogate them.
AV Club gave "Hummels & Heroin" a "A-" rating saying: "Outside of the episode’s general concept, South Park doesn’t have anything hyper-specific to say about the opioid crisis. And the message that is there is fairly obvious: 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. That generalization is fine by me. 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."
IGN gave "Hummels & Heroin" a "5.8" rating saying: "South Park may be on better footing this year, but this show is still hit or miss with its storylines. "Hummels & heroin" was definitely a miss, failing to offer the same insightful look at the struggles of South Park's elderly citizens we've seen in past episodes. This episode still had some strong moments, especially where the musical numbers were concerned, but it still serves as a low point for Season 21 so far."