Lodged between France and Switzerland lies the Duchy of Grand Fenwick, a tiny monarchy. The local economy is entirely dependent on the one product that Fenwick exports, its trademark Pinot Grand Fenwick wine. However, when an American company throws a copycat product on the market ("Pinot Grand Enwick"), the statelet is threatened by bankruptcy.
To prevent the worst, the Fenwickers devise an ingenious plan: To declare war on the USA, get defeated, and then allow themselves to be reimbursed by American subsidies. Soon, a troop of twenty-four longbowmen in mail armor boards a ship to New York City with the intent to surrender to the American authorities — only to run into unexpected obstacles.
Like the film Dr. Strangelove, the book (and later film) The Mouse That Roared was a farcical look at nuclear politics, though it ended on a far happier note than Kubrick's film. Wibberley wrote four sequels to the initial novel: Beware of the Mouse (1958), The Mouse on the Moon (1962), The Mouse on Wall Street (1969), and, finally, The Mouse that saved the West (1981). Each of them follows the same basic formula — the Duchy of Grand Fenwick triumphs over the powerful nations of the Earth in some improbable way.
The third novel, The Mouse on the Moon, was made into another film in 1963 and features Grand Fenwick winning The Space Race.
The novel and film of The Mouse That Roared include examples of:
- Absent-Minded Professor: Dr. Kokintz has his moments.
- Alliterative Name: The "Tiny Twenty".
- Artistic License Chemistry: The Q-Bomb.
- Credits Gag: The typical Columbia logo is replaced by a live-action model, who runs away, scared of a mouse.
- Doomsday Device: It works in the most effective way: everyone's afraid of it going off... and it's broken.
- Double Vision: Peter Sellers' three characters interact with each other in this fashion.
- Driving a Desk: Numerous examples in the film.
- Eek, a Mouse!!: The film opens with a Logo Joke featuring this gag, with the torch-bearing gal in the Columbia Pictures logo fleeing off her pedestal upon seeing a mouse.
- Empty Quiver: Grand Fenwick manages to steal the Q-Bomb during its invasion of the USA.
- Fun with Subtitles: At one point Tully has a "conversation" with a cheerfully garrulous Frenchman; the audience gets subtitles to learn what the man is saying, but the scene ends with one of his countrymen asking Tully what the man said, and Tully saying in English "I don't know, I don't speak French." This comment is subtitled in English.
- I'm Not Hungry: General Snippet stubbornly insists on the bare minimum requirements for prisoners of war, as stated by the Geneva Convention, all while his men are wined and dined at the grand hall.
- Invaded States of America: Sort of. Fenwick decides to invade the United States in order to have them rebuild their country after surrendering, but...
- Loads and Loads of Roles: Peter Sellers, as was so often the case.
- Love at First Sight: Tully towards Helen in the movie.
- Mad Scientist's Beautiful Daughter: Helen Kokintz, except that her father is not mad. (It should be noted that Helen did not exist in the original novel...)
- Micro Monarchy / Ruritania : Grand Fenwick. Exploring all the possibilities of this setting is arguably the raison d'etre of the Mouse series, making it a candidate for Trope Codifier.
- Mistaken for Aliens: The invading Fenwickians.
- The Noun Who Verbed: The title.
- Placebotinum Effect: Possibly subverted at the end of the movie, as the Q-Bomb's lights flicker on after a mouse crawls out of the "dud".
- Ridiculously Potent Explosive: The Q-Bomb is a nuclear device so powerful it can take out a continent if it goes off, small enough to be carried around like (and is "played" by) an American football with some bits glued to it.
- The Short War: America loses to Grand Fenwick in half an hour, before anyone is even fully aware that there was a war in the first place.
- Springtime for Hitler: The original plot to lose the war. But also...
- In one of the sequel books, The Mouse on Wall Street, Fenwick has become wealthy due to part of the settlement of the aforementioned war. However, the Duchess feels that this newfound wealth is corrupting Fenwick's idyllic lifestyle, so she sets out to lose it all on the stock market by picking stocks at random (by throwing darts at the financial section of the paper). However, when other Wall Street traders notice Fenwick is investing heavily in a particular stock, the traders conclude the Duchess must have inside information and immediately invest themselves, driving the price of that stock higher and earning Fenwick even more money. In the end, she sells off all the stocks for cash, has the cash shipped back to Fenwick, and secretly burns it.
- The Mouse fill-in-the-scenario film series ran on this trope. In The Mouse On The Moon. the prime Minister of the Duchy of Grand Fenwick, desperate for indoor plumbing, tries to milk aid out of the US after their main export of wine has turned explosive by asking for cash for a space program. The US, seeing a cheap way to look like they are helping to make space international without doing something as stupid as helping another nation get an advantage over them in the space race by funding someone competent, gives them a million dollars. Keen to top this, the Soviets send them an old rocket, which the PM plans to turn into a boiler for the new hot water system. The Scientist from The Mouse That Roared discovers how to make an anti-gravity mix out of the explosive wine and without telling the PM that he is really planning to go to the moon, takes off successfully.
- Uncanny Family Resemblance: The movie opening states that if some Fenwickians seem remarkably alike (while showing the Grand Duchess Gloriana, the Prime Minister Mountjoy, and Tully Bascombe, all of whom are played by Peter Sellers, along with a statue of Grand Fenwick's founder), it is because Sir Roger Fenwick "was in every possible way, the father of his country."
- Worst News Judgement Ever: At one point in the movie, a BBC announcer gives the news briefs from around the world, all concerning the US losing to Grand Fenwick... except for the US, where the big news is the latest baseball scores, mis-identified by the BBC news-reader as "The American version of football."