The gang go to visit Hagglethorn Hall, an ancient Scottish castle rebuilt in America. They stumble into another mystery when it turns out that the castle and its moat are apparently haunted and the family, including music legend Davy Jones, is fighting tooth and nail about whether to sell it.
This episode includes examples of the following tropes:
- Chekhov's Gun: Shaggy's hay allergy starts acting up when the gang stops for directions at a farm. Later, when he starts sneezing in the caverns, Velma correctly guesses there must be straw around. This leads them to discover the secret elevator up to Cyrus Wheedly's barn.
- Clashing Cousins: Sir Sedgwick (and his nephew, Davy Jones) are in a harsh conflict with Sedgwick's cousin, the Duke of Strathmore, about whether or not to sell the family home.
- Easily Forgiven: Despite Shaggy claiming he has a criminal's face and he and Fred suspecting him of being the villain, Creech accepts the apology gracefully, saying everyone makes mistakes.
- Freudian Slip: When Fred persists in asking for directions to Hagglethorn Hall despite the warning, Cyrus Wheedly says that it's the end of the line before correcting the last word to "lane." After the reveal that he played the Moat Monster, it's clear that he pretended to mix up his words.
- Frog Men: The Moat Monster, which is a giant frog.
- Monster in the Moat: In addition to the eponymous horseman, there was a giant frog-like creature in the moat, aptly referred to as The Moat Monster. Naturally, it's all part of a "Scooby-Doo" Hoax.
- Music Soothes the Savage Beast: When Davy, Daphne, and Velma find the Moat Monster blocking their way, Velma suggests he sing to it, name-dropping the trope. This leads to a chase scene backed by Davy singing "I Can Make You Happy" while he and the girls try to evade the monster.
- Mythology Gag:
- When Velma suggests he try to soothe the Moat Monster with a song, Davy protests that he's never sung for frogs before, only monkeys (presumably referencing the band that made him famous).
- The song Davy sings during the chase scene ("I Can Make You Happy") appeared in an original series chase scene during the episode "Mystery Mask Mix-up."
- The Punishment Is the Crime: After getting caught, villains-of-the-week the Duke of Strathmore and Cyrus Wheedly end up having to dress up as the Haunted Horseman and Moat Monster in a form of Kayfabe, effectively making this one of the few times a "Scooby-Doo" Hoax was done to the public. Each of them gripes about the role.
- Red Herring: Creech the butler looks and speaks like someone out of a horror movie. Shaggy even voices his suspicion that he's "haunting" the castle. However, he rescues Shaggy, Scooby, and Fred from the Horseman and willingly pitches in with the castle's new tourism business after the mystery.
- Reed Snorkel: At the end of the chase scene, Davy, Daphne, and Velma temporarily ditch the Moat Monster by hiding in a pool, breathing through three straws.
- Right Behind Me: Shaggy declares that he is suspicious of Creech the butler, saying that he has the face of a criminal right as Creech comes in behind him and frowns at this.Shaggy: I'll bet he's behind this.Davy: Well I know one thing, he's behind you.Shaggy faints.
- Series Continuity Error: Shaggy is shown to have a hay allergy, which contradicts several past and future episodes, most obviously "The Frickert Fracas", which is part of the same season.
