After seeing the sights in historical Boston, the gang is off to pay a surprise visit to Shaggy's Great Uncle Nathaniel. On the way they stumble upon The Harlem Globetrotters, who are picnicing and trying to rest up after a string of games they played around Boston. Back on the road to Uncle Nat's, Shaggy takes control of The Mystery Machine, and they begin to encounter ghosts resembling Paul Revere and soldiers of war in the road along the way. When the gangs get to Uncle Nat's, he reveals that the ghosts have scared everyone away -- and they're doing they're best to try to scare him away too. Soon the kids, Globetrotters and Uncle Nat begin running into ghosts all over the house. The following morning, everyone is on their way to a local cove when Uncle Nat tells them to keep their eyes peeled for a sea serpent that's been spotted in the lake -- subsequently Shaggy, Scooby, Meadowlark, and Curly encounter the the monster while scuba-diving. Fred and Velma come to the conclusion