The economy of the teeny-tiny European duchy of Grand Fenwick is threatened when an American manufacturer comes up with an imitation of Fenwick's sole export, its fabled wine. Crafty prime minister Count Mountjoy (Peter Sellers) comes up with a plan: Grand Fenwick will declare war on the United States.
Grand Duchess Gloriana (Peter Sellers again) is hesitant: how can meek little Grand Fenwick win such a conflict? Mountjoy explains that the plan is to lose the war, then rely upon American foreign aid to replenish Grand Fenwick's treasury. Bumbling military officer Tully Bascombe (Peter Sellers yet again) leads his country's ragtag army into battle.
They cross the Atlantic in an ancient wooden vessel, then set foot on Manhattan Island, fully prepared to down weapons and surrender. But New York City is deserted, due to an air raid drill. While wandering around, Sellers comes upon atomic scientist David Kossoff and the scientist's pretty daughter Jean Seberg.
Kossoff has been working on the deadly Q Bomb, a football-sized weapon with the destructive capacity of a hundred hydrogen bombs. Suddenly seized with patriotic fervor, Tully captures Kossoff, his daughter and the bomb and brings them all back to Grand Fenwick. Tully has won the war-precisely what he'd been told not to do.
The upshot of this victory is that every world power converges upon Grand Fenwick to claim the Q Bomb for themselves. The satire is heavy-handed at times, but The Mouse That Roared contains several unforgettably hilarious moments, including one startling false ending.
One of the best gags involves the Columbia Pictures logo--a bit frequently cut from TV showings, worse luck. Based on one of the many Grand Fenwick novels by Leonard Wibberly, The Mouse That Roared was a success, yielding a Peter Sellers-less sequel, 1963's Mouse on the Moon.