Not the Nessie

So the story takes us to a certain lake in Scotland, and our adventurers/detectives/four meddling kids and an animal find themselves investigating the legendary Loch Ness Monster. Frightening, water bound and usually fog-shrouded encounters with that legendary beastie follow, and at the denouement we find the mystery solved!

It wasn't a lake monster at all, but a disguised submarine.

We don't see this trope much today, but submarine masquerading as Nessie to scare people away is the solution offered in a number of older films that feature that world famous lake monster. Variations include robot Nessies, paper mache monsters, and other counterfeits but the most often encountered version is a submersible ship, with a gargoyle head attached to give that scary sea serpent effect.

In these instances, the writers and producers were sheepish about introducing a real Loch Ness Monster into the world of the film. A a sub with a monster head riveted onto it is the default answer to the enigma of Loch Ness might seem cheesy. It is a handy way to write off a Special Effects Failure, when it turns out it was SUPPOSED to be fake all along.

There are two sources of this trope: the most well-known fake Nessie photo in history was, in fact, a toy submarine with a monster head; the other that the Nautilus from Twenty Thousand Leagues Under the Sea, was initially misidentified as a sea monster.

This is a sub-trope of the Scooby Doo Hoax; the specific solution of a strangely-decorated submarine sets it off as a variant peculiar to that highland lake monster.

Also expect the real Nessie to show up at the end, when nobody's looking.

Anime and Manga

 * This one is used in a very... out-there way in a Lupin III episode. Fujiko's singing attracts the Loch Ness monster. Later on, when Lupin goes out onto the Loch, a Nessie-shaped submarine draws him in. The reason the sub was built? To catch the real Nessie - the scientist who built it has an Ahab complex towards the Loch Ness monster, and uses Fujiko to attract it so he can capture it.

Arcade Game

 * Played with in Dinosaur King. It's not a submarine, it's an Amargasaurus. As per the trope, the real Nessie shows up at the end.

Film

 * The Private Life of Sherlock Holmes is probably the defining version of this trope. In this film, the Great Detective's brother, Mycroft, is directing top-secret development of Britain's first submarine in Loch Ness; an attached prop monster head keeps the local gentry fooled into thinking it's the beastie. Given, of course, that it's just a prop in a studio water tank, the first appearance of this thing still scared the haggis out of this troper when he first saw it.
 * The Strange Monster of Strawberry Cove from The Magical World Of Disney TV show involves an Expy of Loch Ness and Nessie; when a teacher thinks he saw the local monster and is ridiculed as a result, three of his kid students try to help his case by building a fake monster. Ultimately, it turns out that what the teacher had seen was actually smuggler's boat.
 * And an inversion of this trope is found in the film The Water Horse. Nessie -- or Crusoe, as she's called here -- gets shelled by the British Army. Well, they thought she was a submarine!
 * Also played straight, to a degree. Two old men keep trying to get a picture of Crusoe, but fail every time. So they fake it by putting a boat upside down in the water to produce the famous picture of the neck sticking out of the water. Interestingly, as they're faking it, the real one was right on the other side of a tree, in good view if they could've seen it.

Literature

 * One short story featuring The Saint had the detective investigating a murder supposedly caused by the monster. Turns out it was just a hoax to cover up the real murderer... who ends up killed by the real Nessie in the end!
 * Lest we forget, in the original 20,000 Leagues Under The Sea, many early accounts of the Nautilus's attacks speculated that it was a giant narwhal. Certainly the Ur Example of this trope in fiction, as nobody was writing about submarines before Verne.

Live Action Television

 * The Voyage to The Bottom of The Sea episode "Secret of the Loch". The Seaview not only finds a navigable underwater passage into the Loch, but a Soviet -- ah, I mean, Enemy-- submarine there, pretending to be the monster. And based on what the model work was like, doing a rather bad job of it. Some may feel quite cheated to find out Nessie was a submarine in this one and, on considering how often "Voyage" had REAL sea monsters on the show, then feel REALLY cheated.
 * Stingray, a TV series that had actual sea monsters in it, sent Troy Tempest and his crew to Loch Ness, where they found Nessie was a robot, built in the 1940s and put through its paces to again, encourage tourist travel to the loch.
 * The X-Files episode "Quagmire" has Mulder and Scully investigating reports of a lake monster. In one scene the owner of the local tourist trap is shown faking some monster footprints, whereupon he gets eaten by the real monster, a large alligator. Of course, this being the X-Files, we see at the end there really is a giant serpent in the lake.
 * The Goodies follows the basic outline, but the monster is only shown to be fake long after it's captured, when a zookeeper talks about breeding it with another captured sea monster. A real Nessie also hatches from an egg after the zookeeper tried to kill himself in shame.
 * Done on Sea Hunt, naturally enough, although it was only supposed to be a generic "sea serpent," and not Nessie herself.
 * This example takes place in Minnesota rather than Scotland, but on an episode of Little House on the Prarie the children of Walnut Grove fabricate an Expy of Nessie to frighten Mrs. Oleson into leaving an old woman's lakeside house alone after the former tried to take it from the latter.
 * In the Doctor Who serial 'Terror of the Zygons', the Fourth Doctor and his current companions arrive in Scotland and trace an alien signal to Loch Ness, where strange things have been happening recently. Don't worry, it's not Nessie... it's just a giant plesiosaur-shaped cyborg that provides food for the Zygons. Much more sensible.

Newspaper Comic

 * The Newspaper Comic Alley Oop is about a time traveling cave man named Alley Oop. A few years ago, Alley's pet dinosaur Dinny tagged along for one adventure, and the pair wound up in medieval Scotland. Suffice it that the Scots have remembered Dinny's visit ever since.

Video Games

 * Phoenix Wright Ace Attorney uses this one in the fourth case of the first game, with a creature named "Gourdy" being spotted in Gourd Lake. The monster's "head" that was captured on a photo was actually an inflatable Steel Samurai being blown along by the air tank that some idiot (Read: Larry Butz) was using to inflate it.
 * Played straight, then subverted in Mega Man Star Force 2. Geo goes to a foreign country where there is a legend of a monster called Messie. it is a submarine, but the director making money off of a 'Search of Messie' show runs into Hyde, who offers him a sea monster EM partner, Plesio, which he uses to become Plesio Surf.

Western Animation

 * Scooby Doo and the Loch Ness Monster gives us three Nessies: a robotic submarine version, a less sophisticated (but still somehow totally convincing in appearance) "parade float" version, and the real monster. The real one looks exactly like the impostors.
 * Gargoyles had an identical plot.
 * The Monster Lake episode of Inspector Gadget had a robotic Loch Ness monster.

Real Life

 * Zoo education director John Shields once pranked some Nessie-hunting colleagues by planting the doctored carcass of an elephant seal, recently expired of natural causes at Flamingo Park Zoo, floating offshore in Loch Ness.