When Cassidy Williams is continually attacked by deep-sea killer robots, the Mystery Inc. decides to help her unravel the mystery of who is sending these robots and why. This is going to call for a little deap-sea adventure! They enlist the aid of Tom and Tub, two deep-sea diver kids with their own submarine: Moby Dick and their very own sea lion named Scooby! The kids take our gang and Cassidy deep into the Midnight Zone -- under the sea's surface where sunlight never reaches. Here, they find the abandoned Missionary town that slid into the sea some 300 years ago. And it's full of killer robots, digging for something. The robots attack the gang, forcing them to take refuge in a big undersea laboratory built long ago by Abigail Gluck - mad scientist and one of Burlington's original gang members. She was searching for the sixth piece of the Planispheric Disc.