Kyle has received a pet elephant via mail order from Africa. The elephant is much too much big for Kyle to keep in the house, so he tries to bring it to school. Cartman mentions that he got a pot-bellied pig which is small enough to keep in the house. Mr. Hat teaches the class about genetic engineering. Kyle asks if an elephant using genetic engineering can be made smaller, Mr. Garrison supposes that it could be done. Kyle plans to cross breed his elephant with Cartman's pig, but faces a challenge from fellow student Terrence who believes that he could clone an entire human being before Kyle can breed an elephant with a pig. Both are allowed to use this for their science fair projects. Mr. Garrison asks Stan about his black eye, but tells him not to waste his time with "pansy little foo foo problems" when Stan admits that his sister Shelly gave him the black eye. Back at home, Stan suffers more abuse from his sister. The boys visit the South Park Genetic Engineering ranch where Dr. Mephisto tells them that what they want to do is impossible but at the same time he swipes a sample of Stan's blood.
Terrence has cloned a foot and plans to have a whole person by Friday. The boys ask Chef for advice. He tells them that all they really need to do to cross breed the elephant and the pig is to have them make sweet love to each other. Stan suffers more abuse from Shelly. The boys try getting the elephant and the pig drunk, but it takes Chef and Elton John to set the mood. An elephant makes love to a pig.
Back at the South Park Genetic Engineering ranch, Terrence and his father (Dr. Mephisto) have created a human big-headed clone of Stan who escapes and goes on a rampage. Dr. Mephisto warns the boys about the escaped big-headed clone. The big-headed clone is on the rampage in downtown South Park. Stan gets the big-headed clone to come home with him, so that he can take his revenge on Shelly. Dr. Mephisto reclaims his clone and Stan is worried he is going to get into trouble for all the damage his clone had caused. Shelly comes to his defense, only so that she may abuse him further.
When the science fair projects are turned in, Terrence presents a five-assed monkey and Cartman's pig gives birth. Only the result looks nothing like a elephant-pig combination, rather a pig that looks like Mr. Garrison.
Kenny dies when he's sent flying into the microwave by Stan's big-headed clone.