After being jailed for solving another mystery, the Mystery Inc. gang stumbles upon another mystery: a gruesome monster lurking underneath the town and cocooning everyone who comes near in green goop.

The children of Crystal Cove are being turned into monsters by a creature named Que Horrifico, scaring all of the adults in town. As a result, they abandon their children and run for their lives.

Fred becomes the target of a high school crush when new girl Alice May falls for him. However, Alice May is actually a ghost with a reputation for taking guys with her and making them disappear forever.

A monster called the Fright Hound starts attacking Crystal Cove and people believe Scooby-Doo is behind it. Could Scooby be the one attacking the city or is someone setting him up?

The heroes must work together to realize that their love for each other is the true Heart of the Jaguar.