Fergie runs into Cooper Holmes in the woods between their houses while he is out looking for dogs. She had spied on the Holmes family moving in and had met Cooper's brother Mickey earlier. Mickey had told Fergie that he and Cooper constantly play pranks on one another, and he convinced Fergie to scare Cooper into thinking their house and the woods surrounding them are haunted.
Fergie feels bad when she sees that Cooper really believes that the woods are haunted and admits to her scheme. She and Cooper become friends after she apologizes. Fergie and Cooper bond over how to scare Cooper's brother Mickey to get back at him.
Fergie and Cooper investigate two ghost dogs, eventually following them into the woods at their urging and discovering a shack called the Changing Room. The ghost dogs were not dogs at all, but humans who were cursed to roam the woods as dogs. By Fergie and Cooper entering the Changing Room with them, they provided a way for the humans stuck as dogs to escape. They switch bodies with Fergie and Cooper.
As dogs, Fergie and Cooper try to convince Cooper's parents that they are the real Fergie and Cooper, but are unsuccessful. They eventually are successful in dragging the fake Fergie and Cooper back to the Changing Room, but two chipmunks had entered the shack prior to their getting there, and Fergie and Cooper trade bodies with the chipmunks instead.
Fergie watches as a human who inhibited one of the ghost dogs exits with her body, and Fergie exits the shack as a chipmunk.
Fergie has long, red, frizzy hair that sticks out in "a million directions."
Fergie is friendly and eager to please. She is caring, as evidenced when she regrets scaring Cooper.