Stan and Kyle are playing with their Jennifer Lopez play set (and by playing, they are using a magnifying glass to melt it while Kyle does Lopez' screams of pain) when Cartman comes over to demonstrate something he had learned from the Fifth Graders. The trio then run to Kenny's house where Cartman then begins to masturbate Kenny's dog, which they think is milking it and playing a game called "Red Rocket".
Stan, not knowing what masturbation really is and thinking that what Cartman did was cool, demonstrates "Red Rocket" on his dog Sparky in front of his parents' company (the Stotches, the Blacks, the Tweaks and the Tuckers). His parents see it and are horrified. Stan is then sent to his room, his aroused dog following after him. Stan is then grounded for 10 months which was then reverted pretty soon after.
Stan does not understand why he is being punished and when his parents try to explain to him what sex is, they don't really know how to explain it. They then decide with other parents that the schools should be teaching sex education at a younger age, apparently forgetting about Stan's punishment.
Mr. Mackey and Ms. Choksondik instruct the fourth graders on sex-ed and while Miss Choksondik is teaching the girls about the perils and horrors of sexually transmitted diseases and pregnancy, Mr. Mackey tries to explain to the boys how to have sex. Not remembering what to do from his last encounter, at age 19, Mr. Mackey comes up blank, while Miss Choksondik tells the girls that they will catch an STD and die if the boys are not wearing condoms. Meanwhile, Mr. Garrison is tasked with teaching proper condom use - to kindergarten children. His preferred method of applying one using the mouth shocks his students. As does his run-off list of all the different and possible sexual positions.
The boys, not having learned anything, ask the girls what they learned. The girls, still horrified from their lesson with Miss Choksondik, back away and tell the boys that they need to wear condoms at all times. The boys then go to the pharmacy where the male pharmacist refuses to sell the boys condoms. However, the female pharmacist is on the boys' side and sells them the condoms because she wants them to be safe if they are in fact having sex.
The boys then buy the condoms and wear them at all times, but when they get too uncomfortable, they talk to Mr. Mackey who informs them they only have to wear them when they are having sex. Angry that the girls misled them, they go to a fort where the girls are hiding and tell them to surrender and leave town, a la The Road Warrior. Bebe refuses and throws a boomerang with a sharpened edge at Clyde - who ducks, resulting in the boomerang hitting Kenny, killing him. This subsequently causes Stan and Kyle to order an attack.
While Miss Choksondik and Mr. Mackey are preparing the lesson plan, they swap stories about their sexual past, both come up with sad experiences and hardships involving sex and adolescence itself and they become interested in each other. The two then undress, kiss passionately and then have a rather grotesque, yet touching sexual encounter.
The battle between the boys and girls leads to several casualties - rapid fire nail guns and Molotov cocktails are used in the battle. Eventually, the girls' compound is destroyed by the boys, in an explosion that everyone in South Park feels. Cartman comments that Butters is in big trouble now. Sharon is quick to blame Stan, but Ms. Choksondik then mentions that she forgot to tell the girls that to get an STD, intercourse is required.
After this, Chef explains that sex is emotional and needs to be taught by family, rather than at schools through textbooks and diagrams, since they could end up being taught by someone who doesn't know (Mr. Mackey), someone with a bad opinion of it (Ms. Choksondik) or a pervert (Mr. Garrison). When asked when children are ready to have sex, he responds with an unconditional, "17 — you're ready," a reference to the age of consent in Colorado.
All of the children are relieved they don't have to worry about disease for at least 8 years (especially Stan and Wendy, who are in love), and the episode ends with Cartman playing "red rocket" with another dog. They all get together, smiling. The children are free to go back to their naive, innocent ways (except for Kenny, who was already a bit of a pervert).