In 1942, a wisecracking New Orleans private eye is hired by 3 European powers to stop a blackmail scheme by the most hated man in the world: J. Edgar Hoover.
In 1942, blackmail demands are delivered to representatives of the British, the French and the Germans (oddly enough, Adolph Hitler’s not in power). The delivery to the British government goes not to Winston Churchill, but Neville Chamberlain. The documents threaten to expose certain high officials in an embarrassing sex scandal.
Chamberlain, consulting with King Edward and his official consort, Lady Wallis Simpson, decides there’s only one man that can help them. That man is Arthur Port, who is seen thwarting two bullies trying to rough up a blind black piano player at Arthur’s favorite New Orleans bar. The bullies turn out to be FBI agents. Arthur leaves the two agents barely conscious and leads the blind pianist, young Ray Robinson, to the safety of Arthur’s apartment.
There, he finds a representative from the British government with a heartfelt letter from Lady Simpson, but Arthur is not inclined to take the job. He runs into the two FBI agents again, and after another brawl, they warn Arthur to stay out of the Bureau’s business.
That’s all Arthur needs. He takes the job to show that he’s not afraid of them.
Arthur and Sam, the daughter of Chicago mobster Sal Sarzana, soon discover the FBI’s using a secret lodge in Alsace-Lorraine to photograph European officials in flagranté dilecto. Further investigation in London leads them to an English official who’s an unwilling Hoover accomplice: Churchill, out of power and heading up the loyal opposition. Arthur’s determination to trap Hoover convinces Churchill to help. He offers them an experimental long-range Spitfire to get them back to the States without FBI interference. Arthur’s fancy flying allows them to ditch pursuing American planes over the North Atlantic.
They travel to the trendy Georgetown neighborhood and photograph Hoover’s wild goings-on, complete with formally dressed and decidedly un-feminine guests. But Arthur has second thoughts about using the pictures to blackmail Hoover, admitting he’d be no better than them.
Despondent, they fly back to London, prepared to tell Chamberlain they’ve failed. But as Sam leafs through a forged Gutenberg bible in Chamberlain’s office, Arthur has a bolt of inspiration. He quickly organizes support from the English, French, and Germans, and prays that it all comes together in the time they have left.
They hurry back to New Orleans, where Arthur calls on an old friend, Paul Kacoe, once the best forger in the States, but now a well-respected painter. Arthur explains his plan: Paul will make a forgery of a lost Journal from the Louis and Clark expedition, and Arthur will get Churchill to inform Hoover it’s being auctioned by the French to cover their blackmail costs. After Paul creates an excellent forgery, Arthur rips out the last page before pronouncing the finished book “a masterpiece.”
With England’s help, the Journal is offered at the next Sotheby’s auction, where Hoover’s agents take the bait and steal the Journal. Later that day, Hoover donates the Journal to the Smithsonian, where it’s displayed under bullet-proof glass.
Back at his office, Hoover gloats in victory, until he notices Arthur and Sam waiting in the shadows. Hoover boasts about his triumph until Arthur plays his last trump: a photograph of the ripped-out page. The handwriting on the back of the page states that, in return for bringing the copied Journal to safety, the Louis and Clarke expedition would reaffirm French sovereignty over the part of the Northwest Territory that the U.S. didn’t buy in the Louisiana Purchase. Arthur quips that the French would probably be able to pay the blackmail money if they owned Portland and Seattle.
Later, Arthur releases one last bombshell: they didn’t catch the real mover behind the plot. Sam’s surprised to hear the name, Churchill! Arthur explains their involvement was all orchestrated by Churchill to get out from under Hoover’s thumb. But since they stopped Hoover, Arthur and Sam accept being played for fools.