The novel originally appeared as a six-part serial in The Saturday Evening Post from 25 December 1954 through 29 January 1955, under the title The Day New York Was Invaded.

Landing in New York City, almost completely deserted above ground because of a citywide disaster drill, the Duchy's invading "army" (composed of the Field Marshal Tully Bascomb, three men-at-arms, and 20 longbowmen) wanders to a top secret government lab and unintentionally captures the "Quadium Bomb" (a prototype doomsday device that could destroy the world if triggered) and its maker, Dr. Kokintz, an absent-minded professor who is working through the drill.

The invaders from Fenwick are sighted by a civil defence squad and are immediately taken to be "men from Mars" when their chain mail is mistaken for reptilian skin.

With the world at the tiny country's mercy, Duchess Gloriana, the leader of Grand Fenwick, lists her terms: all the nuclear weapons of the powerful nations must go through an inspection by impartial scientists.

Kokintz takes on his new role as scientific director of the "Tiny Twenty", a new superpower of 20 of the world's smallest nations headed by Grand Fenwick.

"[5] The Mouse That Roared was made into a 1959 film starring Peter Sellers in three roles: Duchess Gloriana XII; Count Rupert Mountjoy, the Prime Minister; and Tully Bascomb, the military leader – and Jean Seberg, as Helen Kokintz, as an added love interest.

Other cast members included: William Hartnell as Sergeant-at-Arms Will Buckley; David Kossoff as Professor Alfred Kokintz; Leo McKern as Benter the opposition leader; MacDonald Parke as General Snippet; and Austin Willis as the United States Secretary of Defense.

In this version, Dr. Kokintz is a physics professor at Columbia University and the arrival of Tully Bascomb's invasion force coincides with a campus student protest.