The Duchy of Grand Fenwick is a miniscule principality in the French Alps, founded in the 15th Century and currently kept wealthy by its exporting of its speciality to the United States. However, trouble looms when the export, a wine called Pinot Grand Fenwick, is replaced in America's affections by a local imposter with a greater advertising budget by the name of Pinot Grand Enwick and the financial bother that the Duchy gets into is catastrophic for them. If they don't find funds soon, says the Leader of the Opposition (Leo McKern), they will be bankrupt. We are bankrupt, the Prime Minister (Peter Sellers) informs him. What to do? There's only one solution, says the Prime Minister, we shall declare war on America! And lose!
This gentle satire was adapted from Leonard Wibberley's novel by Stanley Mann and Roger MacDougal, and is still fondly thought of today. The less charitable would voice the opinion that gentle comedy is another way of saying, "Not funny", and sadly, it's true that The Mouse That Roared doesn't raise too many laughs. However, what it lacks in chuckles it makes up for in charm, and it has the novelty value of Sellers skillfully adopting three roles, reputedly in imitation of his hero Alec Guinness in the Ealing comedy Kind Hearts and Coronets.
Not only does Sellers play the Prime Minister whose idea it is to invade the U.S.A., but also the widowed Duchess Gloriana, the head of state, and Tully Bascombe, the inept chief constable - presumably the crime rate is so low that his services aren't really needed. He is chosen to lead the nation's army (about twenty unenthusiastic conscripts), and despite his protests that he'd rather stick to birdwatching, Tully and company are sent off on a bus to Marseilles (one eccentric joke has a caption with the pronunciation of Marseilles followed by the pronunciation of Washington).
From the French port it's a trip over the ocean on a small boat to North America, and here the plot takes the form of a "fancy that!" story buried on page ten of the daily newspaper. When the Grand Fenwick army arrive to invade, they find New York deserted due to a military manoeuvre testing the capacity of the Q-Bomb, a weapon supposedly many times more powerful than the A-Bomb. So with nobody to stop them, the Europeans wander around until they find the scientific institute where Professor Alfred Kokintz (David Kossoff) is tinkering with his creation instead of taking cover. Now Grand Fenwick has possession of the Q-Bomb.
And therefore, have won the war, the point being that whoever has the most destructive technology, no matter how small the country or group of individuals, has the most say in how the world is run. The soldiers kidnap a general, some policemen, the professor and his daughter, Helen (an imported Jean Seberg) and take them, with the bomb, back to Grand Fenwick. Although this is supposed to be a satire on the powermongers of this world, you get the impression that the British film makers and their handful of U.S. cohorts really, really love America and would dearly want their film to be a big success there. An awkward romance strikes up between Tully and Helen, and the story ends with a chase, but while this is a pleasant diversion none of it is anything close to hilarious, being better as a concept than in execution. Followed by a sequel, Mouse on the Moon. Music by Edwin T. Astley.