Mr. Garrison suggests the boys use their genetic modifications for the upcoming science fair and that Kyle and crew should check out the genetic engineering ranch outside of town. Mr. Garrison then asks Stan to stay after class to talk about his black eye. When Stan explains that the bruise is from his sister, Mr. Garrison calls him a wimp and sends him home.
Once at home, Shelley again beats up Stan and throws him out the window, where his friends are waiting for him. They head off for the South Park Genetic Engineering Ranch.
While at the ranch, Dr. Mephisto shows them his genetically engineered collection, including several different animals with four buttocks, such as a monkey, ostrich and mongoose. He also has rats spliced with ducks, gorillas spliced with mosquitoes, rabbits spliced with fish (which Cartman claims is just a fish with fake bunny ears tied to itself), and Swiss cheese spliced with chalk and a beard (which is simply the three objects sitting separate in the same cage). Mephisto then explains that, just like the Loverboy song says, "pig and elephant DNA just won’t splice." He then steals a blood sample from Stan and the boys flee.
Back at school the boys learn Terrance has cloned a human foot. The boys go to Chef with their genetic engineering problem, and after he too cites the Loverboy song, he gives them the idea to try to have the animals "make sweet love" to breed. Stan goes home and gets beaten up by Shelley again.
Meanwhile, back at the ranch, Mephisto and his assistant have created a human clone of Stan for Terrance (Mephisto’s son)'s science project. The boys attempt to get the pig and the elephant drunk to get them to mate, but it doesn’t seem to be working until Chef stops by and sings to the animals with a little help from Elton John.
The cloned Stan breaks free from Mephisto's ranch and proceeds to terrorize the town. The boys eventually find the clone and take it to Stan’s house and convince it to attack Shelley, but it instead destroys the house and indirectly kills Kenny. Mephisto shows up and shoots the clone, but Stan is afraid he’ll be in trouble for everything the clone did. However, Shelley, in a brief moment of kindness, takes the blame, after which she beats up Stan again.
When the science projects are due Terrance presents a monkey with five buttocks, but Kyle has nothing until the pig gives birth to a pot-bellied pig that looks like Mr. Garrison. Garrison quickly awards it first prize over Terrance's monkey.