"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 rather 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” (unaware that they were internet predators only interested in him on account of his young age). He gets an overflow of replies and converses with some eager men, potential pedophiles and child molesters who act like they want to be friends with him. The next day Cartman meets Tony316, a very stereotypical pedophile, who gives him candy and backrubs. He later is arrested for soliciting sex from a minor. Cartman doesn't understand what happened and goes back to the chat room to find someone else.
In the subplot, Mr. and Mrs. McCormick are trying to get pregnant again, which doesn't suit well with Kenny. He knows that when the baby sees the light of day, his parents will be give more attention to the baby and he would have to give it his food and take care of it. 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 tries to meet another man that he met online by the name of "Mr. Hammerhead". He meets him at some docks and immediately learns that Mr. Garrison is “Mr. Hammerhead” and 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. At the South Park Police Department, a group called the North American Man/Boy Love Association (NAMBLA) tries to break Mr. Garrison and the other pedophile out, claiming that there's nothing wrong with trying to love young boys. Officer Barbrady ignores their pleas and forces them to leave.
Cartman later visits Dr. Mephesto and asks him to genetically engineer some adult friends for him. Mephesto says he can't, but encouarges Cartman to join NAMBLA because he himself is a member and says Cartman looks about right to join. The next day, NAMBLA holds a meeting at the South Park Inn and decides that they need ways to get young boys to join the association. Cartman shows up and the members gladly accept his request to join, making him their poster child.
Meanwhile, Kenny learns that his mother is pregnant and tries to find ways to terminate the pregnancy. He first tricks a pharmacist into selling him abortion pills, which he then mixes in with vodka into his mother’s favorite drink. Stuart drinks it for her because she isn't allowed to drink while pregnant, and it causes him to have heavy diarrhea. Kenny then sees a commercial for a new amusement park ride called the “John Denver Experience”, which pregnant women shouldn't ride, and takes his parents to North Park Funland to ride it. Despite being violently jerked around on the ride, Mrs. McCormick is unharmed while Stuart ends up getting his nose broken.
While Cartman poses for pictures for NAMBLA, the leader tells him that a huge banquet is going to be thrown in his honor. Cartman is told to invite all the boys in town, but he doesn't invite Stan or Kyle in an act of revenge. He brags about the banquet to Stan and Kyle, who tell him they don't want to go anyway. But when Cartman leaves, Stan suggests to Kyle that maybe they should act more mature and they decide to get into the organization too.
Later, Kenny has a nightmare about the baby being born as a grotesque humanoid who kills the doctor, the nurse and Kenny himself. Frustrated and angry, Kenny finally grabs a plunger and chases his mother all around the town hoping to plunge the fetus out of her body while Stuart follows to stop him.
The police raid a meeting of Dr. Mephesto's organization, but it turns out there are two NAMBLA’s (this group is the “North American Marlon Brando Look Alikes,” who are not affiliated with the "North American Man/Boy Love Association"). Garrison is in attendance and gets 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, (Stan and Kyle having been invited as well) the NAMBLA members dance with them and 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. Kenny, still chasing his mother, and Stuart, chasing him, join in the scene too. The boys hide in one of the rooms and Cartman persuades Butters into being their sacrificial lamb by letting the pedophile NAMBLA members have their way with him so the rest of them can get away, to which he reluctantly agrees. Fortunately for Butters, as he looks for the NAMBLA members, he enters an empty room and escapes unharmed. Unfortunately for Stuart, as he looks for Kenny, he goes into the room where the NAMBLA members are and they proceed to have their way with him after they mistake him for one of the boys.
In the end, Stuart ends up having to be hospitalized because of the injuries he received during the anal sex orgy and 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; they say they believe in tolerance and equality for all, but pedophiles are criminals who hurt young people and should be treated as such. After being forced to apologize to his friends for what happened, Cartman decides that he doesn't need adult friends and is willing to wait until his friends become mature like him.
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 incorrectly says it's the 52nd.