When word gets around that McClure's magazine is about to publish an expose on the alleged death of Old Red Amlingmeyer's hero, Sherlock Holmes, Old Red and his brother Big Red are alarmed. They consider Arthur Conan Doyle's famous fictional detective to be a real person, and anything written to the contrary about him gets them riled up. Big Red mourns the loss of his hero and states that any claim of Holmes being a fictional character is "scandalous, slanderous, preposterous and just plain dumb."
In response to the article, the World's Columbian Exposition of 1893 that is taking place in Chicago has decided to invite renowned detectives from around the world to participate in a World's Greatest Sleuth contest. Much to their surprise, Otto "Big Red" Amlingmeyer and Gustav "Old Red" Amlingmeyer receive an invitation from a publisher by the name of Urias Smythe to come to the "White City" and participate in the competition.
In the same way that Dr. John Watson documented the many adventures he had with his friend and partner, Big Red has penned a number of stories based on the mysteries he and Old Red have been involved in. They're known for being quite colorful with amusing chapter headings in every story. Their tales focus on the "deducifyin'" skills that Old Red utilizes in a manner patterned after Sherlock Holmes.
When the brothers arrive at the Exposition, they find that they are in the company of many internationally known detectives: Eugene Valmont from France; Boothby Greene from England; King Brady from the US; and a woman whom the brothers know quite well, Diana Crowe. Each sleuth is accompanied by a publisher or confidante --- in the case of Diana, she is paired with her father, Colonel C. Kermit Crowe. The contest is overseen by the famous William Pinkerton, and the daily scavenger hunt mystery challenges are put together by puzzle-master Armstrong B. Curtis, Esquire.
On the first day of the contest, Big Red and Old Red come up short. They are not as concerned with losing the first challenge as they are with the bearded man who seemingly interfered and caused them to lose. The Amlingmeyers become very suspicious about their fellow contestants in addition to their sponsor. However, the second day of the festival turns up more than just the daily prize in the challenge. The brothers actually locate the prize for day two within a giant hunk of cheddar cheese, yet they also discover Curtis's body, face down on top of the cheese. Old Red deduces that, since cheddar is hard on top, this could not have been an accident --- someone had to have forced Curtis face down into the cheese chunk to smother him to death.
The contest continues, but Big Red and Old Red are far more concerned with sorting out any clues that will lead to the more important task of solving a real murder. Without being able to trust anyone involved, their case is impeded at all turns. Their only ally in whom they can have faith is Diana. The Amlingmeyers and Diana find themselves crossing paths more than once with Chicago Detective Sgt. Moses "Mo" Ryan. Does Detective Ryan suspect the brothers of being involved in the murder, or is he being paid off by the actual killer to impede their own investigation? When a second bearded man appears and interferes with their "detectifying," the Amlingmeyers begin to feel that their own lives m