system
Given the following post, respond as if you're commenting on Reddit:
human
[Scooby-Doo] Has There Ever Been A Mystery The Gang Couldn't Solve? Do They Have Any Cold Cases?
gpt
I believe the film *The Thirteenth Ghost of Scooby Doo* deals with Fred and Velma joining the others in (a few years later, in-universe time) tracking down the ghost that was never actually found during the series *The 13 Ghosts of Scooby Doo*. 

With the series being one of the ‘magic and ghosts are real’, and Velma being a ‘magic isn’t real, and I’ll prove it’, in the movie Velma was trying to use logic and reasoning to explain everything away.