Rupert's day of fishing with his parents is interrupted when the Professor shows up with his latest invention — a small submarine called the Goldfish. Naturally, Rupert goes along with him to explore Nutwood Lake, and it's not long before he sees the last thing he expected to find: a real sea monster! It's none other than Nessie herself, on holiday from Loch Ness. Unfortunately, she's not alone. Robbie McRobbie, a Scotsman who's been hunting her for years, determined to catch her and make a fortune showing her to crowds, is hot on her trail. When he manages to capture her, it's up to Rupert and the Professor to find them and save her.
First appearance of Bill's baby brother Toby.
This episode provides examples of:
- Delayed Reaction:Bill: What are you looking for?Rupert: A sea serpent.Bill: Oh. (beat) A sea serpent?!
- Evil Cannot Comprehend Good: It never occurs to McRobbie that the Professor is lying about only wanting to make money off the Loch Ness Monster himself; thus, he doesn't realize he's being tricked.
- Framing the Guilty Party: McRobbie really did have a sea monster he was charging people to see, but Rupert and the Professor convince them it was a hoax using a submarine so that no one will believe him if he tried that again, so, therefore, he has no reason to try to catch Nessie again.
- Graceful Loser: McRobbie ultimately accepts that Rupert beat him "fair and square," even shaking his hand and thanking him for his suggestion for his next business.
- Guile Hero: The Professor brilliantly convinces McRobbie that he only chased him back to Scotland because he's interested in exploiting the monster for fame and fortune, too.
- Heel–Face Turn: Rupert successfully convinces McRobbie he'll be better off selling souvenirs about the legendary Loch Ness Monster instead of holding her prisoner.
- It Was Here, I Swear!: No one in his audience is interested in hearing McRobbie insist he really DID have a monster in that tank, not a submarine!
- The Masquerade: Rupert initially plans to bring the police after McRobbie, but Nessie is horrified at the thought of the world knowing she's real for a fact because she'll never get a moment's peace and have to leave her home to either hide or stay on the move forever. Rupert finds a way to save her without exposing her.
- The Maze: The underwater connection between Loch Ness and Nutwood Lake is part of a maze of tunnels. Nessie's fellow underwater dwellers guide Rupert and the Professor down the right course.
- Not the Nessie: Discussed, then Invoked — When the Goldfish first runs into McRobbie's sub just after Rupert claims to have seen a sea monster, the Professor assumes it was the submarine he saw (there's absolutely no doubt for Rupert or the viewer that this is ridiculous). Later, Rupert and the Professor ruin McRobbie's exhibit by replacing Nessie with a submarine, causing his audience to take their money back and leave in outrage at the hoax.
- Remember the New Guy?: Justified in Toby's case. Since he's just a baby, it's very likely he and Bill simply weren't old enough until now for Bill to watch him on his own, hence why the viewer haasn't seen him around Bill before now.
- Stock Ness Monster: Female, sapient, and speaks English.
- Sure, Let's Go with That: When McRobbie mentions the tunnels under Nutwood Lake, the Professor assumes he's mapping the network of tunnels. McRobbie quickly says he's right.
- We Need a Distraction: While Rupert works on freeing Nessie, the Professor convinces McRobbie to take him on as a partner and discusses all the plans he has for studying the monster and opening a research facility that will make him even more money.
- Would Hurt a Child: McRobbie is perfectly willing to tip over a boat, dumping 2 boys and a baby into a lake, if it's in the way of his pursuit of Nessie.
