In this origin story about Scooby-Doo (voice of Frank Welker), we see that he met Shaggy on the beach, when he was a pup and Shaggy was a kid. Shaggy quickly adopted him and when the two go trick or treating on Halloween, they meet Fred, Velma and Daphne.
The group forms a detective agency known as Mystery Inc. Fast forward a few years later and Scooby and the gang face their biggest, most challenging mystery ever: a plot to unleash the ghost dog Cerberus upon the world. As they race to stop this global “dogpocalypse,” the gang discovers that Scooby has a secret legacy and an epic destiny greater than anyone imagined, especially when Shaggy and Scooby are kidnapped and find themselves on the Falcon Fury. Back home, the rest of the gang tries to solve the mystery.