The Scooby-Doo Show

A Highland Fling with a Monstrous Thing (3x6)

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The gang heads to Scotland to help Velma's friend Aggie MacDuff, who's ancestral castle home, which she's turned into a tourist attraction, is being haunted by the ghost of her great grandfather Finnyan MacDuff, who in turn has summoned the Loch Ness Monster to help him scare away potential tourists.