Rider Woofson and the other pup detectives are excited to be going on a class trip to Mystery Mountain with the rest of Pawston Elementary. After all, with a name like that, the chances of finding a thrilling new case seem pretty good! But when the PI Pack arrives at the top of the mountain, they discover that all of their classmates have been scared away by a snow monster! Rider and the other detectives smell something fishy and decide it's time to investigate exactly what's so mysterious about Mystery Mountain.