While on a fishing trip, the gang gets detoured into Canada by a flooded road. Their new destination, Bottomless Lake, turns out to have another mystery brewing — a sinister fish monster prowling the depths.
This episode includes examples of the following tropes:
- Border Patrol: The episode has cops patrolling the Canadian-American border and stop the gang at the beginning of the episode while on the lookout for some kind of fugitive robber and also first informing them of the recent monster sighting.
- Chekhov's News: Upon arriving, the gang sees a three-week-old newspaper talking about a gold heist in the Klondike. The villains turn out to be smuggling the gold into the United States using the lake.
- Creepy Gas-Station Attendant: LeBeau the gas station attendant is a hulking, scowling man who warns the gang about the monster and calls them crazy for wanting to stick around as he prepares to flee town. He's innocent, though, and acts nicer in the final scene once the mystery is solved.
- Everyone Laughs Ending: The episode ends with the gang and Mr. Le Beau laughing when Scooby uses the beast costume to scare a bunch of fish and then catches them in a bucket.
- Evil-Detecting Dog: The Johnsons' cat seems pretty friendly to people it hasn't met, given that Velma easily convinces it to curl up in her arms. However, it reacts to Julie Johnson (who turns out to be an imposter and one of the perps of the week) by yowling and running away from her.
- Fine, You Can Just Wait Here Alone: Shaggy and Scooby initially refuse to join the others in scuba-diving in the lake, and Fred, Velma, and Daphne go off without them. When the monster shows itself, the pair hurry to catch up.
- Food as Bribe: When Shaggy and Scooby decline to go to a cabin to ask directions, Fred and Velma change their minds by mentioning how Canadians love giving food to strangers.
- Foreshadowing:
- Julie Johnson says that she returned for her cat, yet when she tries to take it from Velma, it yowls and runs away. This hints that "Julie" was lying and isn't a member of the family.
- The gang hears thunder while camping, yet there isn't a cloud in the sky. It turns out to be the sound of plastic explosive igniting as the villains try to make a tunnel.
- Glowing Eyes of Doom: The Beast has huge glowing eyes that can be seen even in the lake.
- Heroes Gone Fishing: The gang is on a fishing trip when they stumble on another mystery. Afterwards, Scooby manages to catch a number of fish by scaring them with the beast costume.
- Inflating Body Gag: Shaggy and Scooby inflate themselves with an air hose to scare the Monster of the Week. This also enables them to float until they open their mouths and let the air out.
- Moose and Maple Syrup: Downplayed. The only part of the episode that comes close to pulling off The Theme Park Version of Canada is the mountie, and the fact that he and a shopkeeper share a slight French accent.
- Samus Is a Girl: The Beast turns out to be "Julie Johnson", or rather a woman posing as the Johnsons' daughter.
- Screw This, I'm Outta Here: After three weeks of monster sightings, the town of Bottomless Lake is abandoned except for one obstinate fisherman, a Creepy Gas-Station Attendant who wants to leave but has car trouble that he's trying to fix, and a young woman who left but returned to look for her missing cat. One of them is lying and the culprit.
- "Where? Where?": When Julie accuses the gang's dog of scaring her cat, Scooby asks, "What dog?"
