The Black Circle is the fifth book in The 39 Clues series and is written by Patrick Carman. This book was released on August 11, 2009. Amy and Dan Cahill, the protagonists of the series, try to follow a Lucian secret. They also encounter the infamous Madrigals.
The book starts with Amy and Dan Cahill getting a telegram in Egypt, saying "Cairo International Airport, locker number 328. 56-12-19. NRR". The night before, the Madrigals had attacked the Ekaterina stronghold in their hotel, so, fearing their lives, they switched hotels. This suggests that someone is tracking them. They leave Nellie Gomez a note and leave. They are closely followed by Hamilton Holt. In the airport, they lose him, but then run into the Kabras. Dan makes a run for it but Ian grabs Amy quickly. She then kicks him in the leg extremely hard. They make it to the locker in time to discover objects in a box left by a mysterious person who goes by the name of NRR. In the box there is a guidebook, disguises, passports, a glass ball, a credit card, a note with random letters on it, and a photo of their parents outside the Russian Embassy.
As with every book in the series, there is a secret code hidden somewhere in the page numbers. The code for this book is in the Cyriillic alphabet. When translated phoneticaly it says "Olivia had another". Olivia being the wife of the Gideon Cahill (Founder of Cahills). "had another" referring to Madeline Cahill. Founder of Madrigals.