After taking a blood sample from Stan, Dr. Mephisto and his assistant creates a human clone of Stan for Terrance's science project. 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 persuade it to attack Shelly, which it does after destroying the house and indirectly killing Kenny. Mephisto shows up and kills the clone, but Stan is afraid he’ll be in trouble for everything the clone did, but Shelly, in a brief moment of kindness, takes the heat (after which she beats up Stan again).
The clone of Stan Marsh has a similar appearance to Stan, but has a larger head with stringy, black hair and an arm that is longer than the other.