A Highland Fling with a Monstrous Thing is the sixth episode of the third season of Scooby Doo Where Are You!, and the thirty-first episode overall in the series. It was later repackaged as the sixth episode of the third season of The Scooby-Doo Show, and the thirtieth episode overall in the series.
The gang travel to Scotland to visit Velma's friend, Aggie McDuff, who lives in an ancient castle that has been turned into a tourist attraction. On the way there, the gang pass by the Loch, where they see a ghostly Scottish highlander playing the bagpipes and a huge Loch Ness Monster in the lake. They stop at a nearby inn to ask directions, where the inn keeper warns them to stay away from McDuff Castle. They ignore his warnings and head to the castle, where Aggie tells them that a monster and a ghost has been scaring all the tourists away. They meet her assistant, Jamie Craigmore, who claimed to have seen the ghost. The ghost happens to be Aggie's great grandfather, Finnyan McDuff.
Scooby-Doo and Shaggy are terrorized by the Loch Ness Monster inside the castle as the rest of the gang appear with Aggie in the same room. They all hear the sound of bagpipes and see the ghost of Finnyan McDuff standing on one of the castle turrets. Fred, Velma and Daphne decide to search the turret for clues with Aggie, while Shaggy and Scooby search the rest of the castle. Shaggy and Scooby find themselves in the library, where they find a note that says, "Switzer and 100 wristwatches", a clue. Meanwhile, Fred, Velma, Daphne, and Aggie search the turret and find a weather report and a roll of wire. They meet up with Shaggy and Scooby, who have been chased by the ghost. Aggie then shows the gang a section of the castle that has been closed off for centuries.
Even though that section of the castle has been closed off, there are footprints meaning someone has recently been there. They come to an old door, where there is fresh oil on the hinge. It is there they discover hundreds of boxes of wristwatches, stamped "Made in Switzerland". They discover what is going on and decide to trap the ghost and the Loch Ness Monster. Scooby begins playing the bagpipes, summoning the Loch Ness Monster, with Shaggy unwittingly on top of the Monster's head. It turns out to be a mechanical monster that is controlled by the sound of the bagpipes. They trap the Ghost of Finnyan McDuff, who turns out to be Jamie Craigmore. Jamie was smuggling in wristwatches and other goods from Switzerland using a minisub under the Loch Ness Monster. He used the legend of Finnyan McDuff in order to scare away interfering tourists.