Who Let the Ghosts Out? is the first book in R.L. Stine's book series "Mostly Ghostly". This first installment introduces us to the characters and the plot of the series. Max Doyle, an unpopular child with a dysfunctional family is introduce to ghost siblings that only he can see. He must help them find their missing parents, and protect them from an animal traveling spirit monster named Phears.
Siblings Nicky and Tara Roland walk home discovering they have a form of amnesia and no recollection of it being late October.