"Cartman Joins NAMBLA" is the fifth episode of Season Four, and the 53rd overall episode of South Park. It aired on June 21, 2000.
Cartman, in search of more mature friends, accidentally joins the "North American Man/Boy Love Association", and puts all the boys of South Park in jeopardy as a result. Meanwhile, Kenny tries to stop his mother and father from having another baby.
Cartman wants to hang out with older males than Stan, Kyle, and Kenny; because he feels he’s too mature for them. So he starts looking through a chat room on the internet, and stumbles upon the topic: “Men Who Like Young Boys”. He gets an overflow of replies.
He converses with some eager men, potential pedophiles and child molesters who act like they want to be friends with him. Unfortunately, Cartman doesn't understand what's happening. Eventually, he winds up contacting the North American Man/Boy Love Association.
In the subplot, Mr. and Mrs. McCormick are trying to get pregnant again, but this doesn't suit well with Kenny, knowing that when the baby sees the light of day, Kenny's parents will be getting more attention to the baby, Kenny's friends will stop hanging out with him, Kenny himself would have to save food for the baby, change the baby's diapers and take care of the baby, he would have bad luck in his lifetime and the baby would be a total embarrassment to him. So while playing catch with Stuart, Kenny deliberately throws the ball into Stuart’s crotch hard enough for him to fall down puking, hoping that this will deprive him of his fertility.
The next day Cartman meets Tony316, a very stereotypical pedophile, who is arrested for soliciting sex from a minor. The next day, Cartman learns that Mr. Garrison was the man behind the name “Mr. Hammerhead”, after he also gets arrested. Cartman assumes that Stan and Kyle have been trying to sabotage his attempts at making new friends, because he is too naive to know what’s really happening.
Kenny tricks the pharmacist into selling him abortion pills, which he then mixes in with vodka into his mother’s favorite drink. Because she isn't allowed to drink while pregnant, Stuart drinks it for her, and it causes him to have heavy diarrhea. Kenny then sees a commercial for the “John Denver Experience”, an amusement park ride which pregnant women apparently shouldn't ride. He then takes his parents there, but Stuart ends up getting his nose broken.
Soon after, he had a nightmare about the baby being born which looks humanoid, who kills the doctor, the nurse and Kenny, and Kenny wakes up. Frustrated and angry, Kenny finally takes a plunger and begins chasing his mother all around the town with it, Stuart following close behind. Dr. Mephesto tells Cartman that he looks about right to join NAMBLA, and encourages him to do so. He takes this advice, and winds up as a poster model for the association. In his honor, a huge banquet is thrown. Cartman is supposed to invite all the boys in town, but in an act of revenge, he doesn't invite Stan or Kyle.
The police raid a meeting of Dr. Mephesto's organization, but it turns out there are two NAMBLA’s (this group is the “National Association of Marlon Brando Look Alikes,” who are not affiliated with the North American Man/Boy Love Association). Garrison is in attendance and angry, realizing that he's in the wrong place. The "Marlon Brando Look Alikes" have been fighting the pedophile organization for the rights over the name NAMBLA. They and the police join together and all rush to stop the other NAMBLA's banquet.
When the boys have all arrived, (for some reason, Stan and Kyle showed up as well) the NAMBLA members take them to their hotel rooms. It’s now that the boys realize what they’re in for, and a chase ensues. Soon the police and the other NAMBLA arrive and start chasing the naked pedophiles. Next, Kenny, chasing his mother, and Stuart, chasing him, join in the scene. Cartman persuades Butters into letting the pedophile NAMBLA members rape him so they can get away. However, the NAMBLA members and Stuart end up alone in a dark room together, and they mistake him for one of the boys and sparing Butters. He ends up having to be hospitalized because of the injuries he received during their anal sex orgy.
In the end, all of the NAMBLA people are arrested. Their leader gives an impassioned speech about rights to all people. But Stan and Kyle call him out on trying to weasel his way out of jailtime by equating pedophilia with being an oppressed minority. By stating that, while they believe in tolerance and equality for all, pedophiles are criminals who hurt young people and should be treated as such.
The ambulance which was taking Stuart to the hospital accidentally runs Kenny over. Afterwards, Mrs. McCormick gives birth to an identical child (parka included), which they name Kenny. Stuart claims this is the 50th time such has happened, and Mrs. McCormick corrects him, saying its the 52nd.