Sherlock Holmes meets amateur sleuth Father Brown at a café in Chicago, and they chat over tea. They are approached by Detective Usher, who brings to their attention a society dinner thrown by famous widower socialite Hannibal Todd, a man whose daughter, Etta, stands to inherit a mighty fortune at the age of 20. The famous London blue-blood, Lord Falconry, was expected to be in attendance at this theme dinner. The last theme was a “Preambulator Parade” that had the men dressed as babies pushed around by their fiancés.

Holmes notices another story: the savage murder of a prison guard followed by the escape of a convict. The body of the guard was found with his head bashed in and his gun stolen. The convict was caught at a place called Pilgrims Pond. There was a message written in blood on the wall. The message stated the act was in self-defense, that he had a gun, and that he’s “saving a bullet for Pilgrims Pond.” Pilgrims Pond is coincidentally on Mr. Todd’s property.

Usher reveals that Todd takes advantage of the working class and he has a great deal of people who would like to see him dead. While at the pond, Usher notices the figure of a ghostly woman in a ragged cloak he is almost certain was Etta Todd. A loud voice could be heard asking who Lord Falconry was and the woman shouted, “Murder!” and Usher left in hopes of not being discovered.

Usher surmises that the convict was actually an aspiring lover to Ms. Todd. He tested a newfangled psychometric machine on the convict. The machine can tell when a person lies. Yet, the convict inexplicably passed the test and beat the machine. And then a woman accused the convict of being depraved murderer Drugger Davis, who walked away from claims that he murdered three people with an undetectable poison.

Just then Holmes, Brown, and Usher are accosted by a man in rags claiming to be Mr. Todd. He demands that Usher stop persecuting his guests. Father Brown states that the themed society dinner was a slum dinner where the finest dress as the lowest and lays out the entire scene.

Lord Falconry is actually Drugger Davis, who was running away from the convict and was mistakenly arrested. The convict timed his escape with the dinner and was prepared to elope with Etta Todd, and thus uncovers the mystery of Pilgrims Pond.

We jump to Baker Street, where Father Brown is now a guest of Sherlock Holmes. The two head to breakfast, where Holmes uses the arrival of Mrs. Wilson as a display of his deductive method of investigation. Sherlock deduces that she is a fishmonger who has traveled to China with an abusive husband, based off the scars on her hands rough from fish scales, a small coin from the Yangtze, and a left ear permanently swollen from her now-deceased husband’s blows.

She presents Holmes and Brown with a newspaper ad and a query that piques their interest: an open position into the mysterious Redheaded League, the only qualifications being the applicant have red hair and be older than 21. Mrs. Wilson, having red hair, would be perfect for this position. She sent her assistant Millie to investigate the opening on her behalf.

Mrs. Wilson arrived to speak to Mr. Duncan Ross, the trustee of the league. She was hired based on the bonafides of her fiery red hair and was sacked shortly after— the Redheaded League being dissolved. Not only that, Mr. Ross’s name was actually William Moreby, a lawyer who had just moved out of what she thought was the League’s office.

Holmes and Brown investigate on Mrs. Wilson’s behalf. They are accompanied by a Detective Jones of Scotland Yard and a Mr. Merryweather, operator of the City and Suburban Bank. They all meet at the sub-basement for the bank. There is a tunnel that can access this sub-basement which stores French money borrowed for repairs to the city of London.

It seems that young Millie, aka Colette Franz, is making an attempt to grab the French gold using Mrs. Wilson’s home as an escape route. Hiring Mrs. Wilson for the Redheaded League was a ruse to keep her from her pawn shop, allowing Millie’s cohorts to tunnel to the bank. The first clue was based on Holmes noticing the wrinkle and stain on her knees she got from digging.

Millie’s gang is rounded up, and Holmes and Brown admire BOTH the methods of intuition and deduction.