What happens when the smallest country in the world declares war on the United States and wins? That is the plot of the gentle satire called The Mouse That Roared. When the United States seems to put the Grand Duchy of Fenwick out of business with a new formula for cheap wine that imitates the Fenwick wine, their best laid plan is to send a detachment of crossbow men to Manhattan with the express point of losing, surrendering and, thus, gaining American aid to save their nation. But, the attack goes wrong. Americans think the United States has been invaded, not by a medieval band of Zouave guards, but by creatures from outer space. The leader of the mission (Peter Sellers) also meets an egghead scientist who has a secret bomb that does give Fenwick the edge in the nuclear arms race. They return to Fenwick with the good news. Back home the Grand Duchess (Peter Sellers) is appalled at victory and the new Q-Bomb. She and the Prime Minister (Peter Sellers) must then find a way to use diplomacy to win the amity of the United States and show the world an example of how to get along.