Callie is a mother of two who, after being evicted from their big city apartment, took her kids Trevor and Phoebe to Summerville, Oklahoma. There, she’d inherited a mysterious farm left to her by an even more mysterious father. Soon, we learn her father was former Ghostbuster Egon Spengler (Harold Ramis) who recently died.
Egon left Callie and her mother (who is never named) early in her life and while Callie always resented him for that, she learns he did it for a very good reason. He moved to Summerville to protect the world from the ghost he had previously found with the Ghostbusters, Gozer.
Callie eventually becomes part of Gozer’s plan to return only to be saved by her kids and the original Ghostbusters. We don’t see what happens after that, but Frozen Empire suggests they’ve moved to New York.