Case History: While fishing at the beach, Shaggy snags and pulls ashore a giant block of ice containing a frozen caveman. Velma recognizes it as a two million-year-old caveman discovered in the Arctic and washed over the side of a research vessel during a storm. The kids take it to Oceanland and leave it with two scientists, Professors Wayne and Engstrom. After they leave, someone turns a space heater onto the caveman; Engstrom leaves for the dockside while Wayne remains behind to work on some research notes.

At the malt shop, Shaggy remembers that he left his fishing tackle at Oceanland, and the gang returns to pick it up. They find the doors ripped off their hinges, Wayne's torn parka, and the caveman thawed out and missing. The phone is broken, so the kids decide to investigate. They follow giant caveman tracks to the dock, where they see Engstrom working a strange device and seemingly talking to a porpoise. He takes fright and flees into a shack when he hears Scooby creeping up on him. When Freddy opens the door to the shack, they discover the angry caveman, who chases them all about Oceanland.

After they escape, Velma finds a map showing the currents off Oceanland and an object anchored offshore. She, Shaggy and Scooby take a boat out, where they find a fishing boat. They are locked inside it with an ice machine that is churning out giant ice blocks full blast. They escape just before being crushed by the ice, only to find that their boat has been scuttled, so they paddle back to shore on an ice block.

Freddy resolves to catch the caveman and has Shaggy and Scooby (dressed as fish) lure the caveman to a spot where he and the girls can drop a net on him. But Daphne falls, pulling everyone else down into the net. The caveman chases them again but is caught when he falls into the tank holding the giant clam. They pull him out and unmask him as Professor Wayne. Wayne had heard about the loss of the Arctic caveman and had constructed his own "frozen caveman" at the fishing boat, with the intent of letting it drift down to Oceanland. After its recovery, he melted it and disguised himself as the caveman with the idea of getting rid of Engstrom and stealing his revolutionary new marine life communicator.

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