At a feast in honor of the appointment of a new constable, an unknown person stabbed the Marquis de Liancourt with his own dagger. A witness was immediately found who accused Mark de Segur of this crime, after which the king had no choice but to send him under arrest to the Black Tower. Soon, Mark still manages to get free, but he only has two long days to solve this crime and clear himself of suspicion, after which he will have to return to his cell and then appear before the royal court. Unraveling this strange case, he begins to understand that the intrigue is not directed against him or even against his grandfather. The conspirators' target is King Joan.