In the meantime, Anthony Cade is given the task of delivering a manuscript to a publisher and returning the letters (on which the manuscript is based) to their owner. The letters are written by Count Stylptitch of Herzeslovakia, a country in uproar over the recent discovery of oil. While staying overnight at a London hotel, the letters are stolen. The thief delivers one of the stolen letters to the home of Virginia Revel, the signature on the letters, intent on blackmailing her. She pays him some money with the promise of more when he brings another letter. However, when she comes home the next day, the thief is dead on her doorstep and Anthony Cade is hovering over him. Cade arranges to have the body discovered elsewhere to avoid a scandal for Revel who proceeds to Chimneys.
Upon arrival at Chimneys, Prince Michael, the heir to the Herzeslovakia throne, is murdered. Cade, whose footprints are found outside the mansion, is a suspect. He comes forward and persuades Scotland Yard's Superintendent Battle of his innocence. In the meantime, he travels to France to track down the real murderer.
The Koh-i-Noor Diamond, stolen from the Tower of London years earlier by a French thief named King Victor, might be hidden in the mansion at Chimneys. One night Virginia Revel comes upon an intruder, and suspects that it is King Victor, who has been released from prison. However, it is M. Lemoine of the Surete, who is searching for the thief.
The stolen letters, which appear in Cade's room, provide a clue for Superintendent Battle: "Richmond seven straight eight left three right." Battle follows the clue to Richmond where he finds a brick in a hidden passage way. Cade heads to Dover where he discovers Hiram Fish, who is not a collector of books, but a Pinkerton detective on the theif's trail, and the real M. Lemoine, who is tied up as a hostage.
At Chimneys everyone reconvenes to hear the mystery revealed. Miss Brun holds a pistol to Boris to retrieve the diamond. It turns out that Miss Brun was the murderer of Prince Michael, who had discovered her secret identity as the last queen consort of Herzeslovakia. In a Princess Anastasia-like twist, she was thought to have been murdered with her husband in the revolution but escaped. She was the one who had written the coveted letters and signed them with Virginia Revel's signature.
Anthony Cade introduces the real M. Lemoine to the group. Hiram Fish captures King Victor who has been posing as a French detective. Anthony Cade gives the letters to Jimmy McGrath and earns 1000 pounds. Cade and Fish follow the code of the letters to a rose on the grounds of Chimneys where they discover the precious diamond. Cade then reveals he is the missing Prince Nicholas, the cousin of the decease Prince Michael. He is ready to ally himself with British syndicate. He offers himself as Herzeslovakia's next king. His queen will be Virginia Revel, whom he married earlier that day.
The Secret of Chimneys circa 1925 courtesy https://en.wikipedia.org/wiki/The_Secret_of_Chimneys#/media/File:The_Secret_of_Chimneys_First_Edition_Cover_1925.jpg.